feat(podcasts): the episodes authoring door + curated chain order

"Newest episodes from my selected podcasts" is now buildable in the UI,
end to end — the user could not find ANY podcast-related filter surface
because none existed: the New Collection modal could only create
book-scope smart collections, and the engine's episodes scope (item 3.1)
had no authoring door.

The scope door:
- The create modal's Smart card gains a Rules-scope segmented control
  (📚 Books / 🎧 Podcast episodes); the copy no longer claims smart
  means "match books by rules". Choosing Episodes seeds an EMPTY
  episodes tree ({"scope":"episodes","mode":"all","children":[]}) so the
  collection is born episode-scoped: the editor opens with the episode
  vocabulary (feed/downloaded/played/published date) and membership
  runs the episode engine from the first rule.
- ParseFilterTree now accepts an EMPTY ROOT (nested empty groups stay
  invalid) — the seed's legal shape. The root arrives at depth 1 by
  call convention, hence the depth>1 exemption.
- The composers define the empty-tree semantics: zero children matches
  NOTHING (ComposedLiveMedia/ComposedCount guards), mirroring
  EvaluateSmartCollection's zero-enabled-rules convention. Without the
  guard the composed query degrades to the bare scope head and a fresh
  collection would swallow every episode in scope — pinned by
  TestEpisodeCollectionsEmptySeedMatchesNothing (envelope book_count 0)
  and TestEmptyRootTreeSeedsEpisodesScope.

The curated chain order (plan §10 R2, user need: the briefing plays
NEWEST-first):
- view_settings.chain_order ("native"/"newest"/"oldest"/"reverse")
  stored on the collection — curated content, not a player-side knob.
- PUT /api/collections/:id/view-settings merges the key into the
  stored JSONB (a blind write would clobber the playlist toggles).
- buildSmartCollectionSequenceContext routes OrderEpisodes through
  handlers.CollectionChainOrder; the detail header's Play-as-Sequence
  gains the order select next to it (PlayCollectionButton), which
  PUTs and reloads. Pinned by chain_follows_the_curated_order
  (reverse flips the chain indexes and hops).

Caught by the new test, a latent clobber fixed: UpdateCollection
treated a NIL view_settings payload as "write NULL" — any edit that
carried no settings dropped every stored key (chain_order, playlist
toggles from API clients, imported provenance). Nil now means
untouched, the same convention filterTree already had.

Composer guards also cover ComposedCount, so the builder's live count
reads the same semantics. Tests: collectionChainOrder unit matrix,
empty-root parse unit pin, two integration pins (seed matches nothing;
curated order drives the chain and survives whole-map edits).

templates/collection_modal_templ.go

@@ -168,14 +168,14 @@ func CollectionModal(collection CollectionData) templ.Component {
 				return templ_7745c5c3_Err
 			}
 		} else {
-			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 18, "<form hx-post=\"/api/collections\" hx-redirect=\"/collections\"><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Type</label><div class=\"grid grid-cols-2 gap-2\"><label class=\"relative cursor-pointer\"><input type=\"radio\" name=\"collection_type\" value=\"static\" checked class=\"peer sr-only\"><div class=\"p-3 rounded-lg border text-center peer-checked:border-blue-500\" style=\"background-color: var(--bg-primary); border-color: var(--border);\"><span class=\"font-semibold text-sm\" style=\"color: var(--text-primary)\">Static</span><p class=\"text-xs mt-1\" style=\"color: var(--text-secondary)\">Pick books by hand</p></div></label> <label class=\"relative cursor-pointer\"><input type=\"radio\" name=\"collection_type\" value=\"smart\" class=\"peer sr-only\"><div class=\"p-3 rounded-lg border text-center peer-checked:border-blue-500 peer-checked:ring-2 peer-checked:ring-blue-400\" style=\"background-color: var(--bg-primary); border-color: var(--border);\"><span class=\"font-semibold text-sm\" style=\"color: var(--text-primary)\">⚡ Smart</span><p class=\"text-xs mt-1\" style=\"color: var(--text-secondary)\">Match books by rules — always live</p></div></label></div></div><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Name</label> <input type=\"text\" name=\"name\" required class=\"input\" placeholder=\"My Reading List\"></div><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Description</label> <textarea name=\"description\" class=\"input\" placeholder=\"Optional description\" rows=\"3\"></textarea></div><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Icon</label> <input type=\"text\" id=\"icon-search\" class=\"input mb-2\" placeholder=\"Search or type emoji...\" maxlength=\"4\" @input=\"filterIcons($event.target.value)\" @focus=\"showAllIcons()\"> <input type=\"hidden\" name=\"icon\" id=\"collection-icon\" value=\"")
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 18, "<form hx-post=\"/api/collections\" hx-redirect=\"/collections\"><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Type</label><div class=\"grid grid-cols-2 gap-2\"><label class=\"relative cursor-pointer\"><input type=\"radio\" name=\"collection_type\" value=\"static\" checked @change=\"setCollectionType($event.target.value)\" class=\"peer sr-only\"><div class=\"p-3 rounded-lg border text-center peer-checked:border-blue-500\" style=\"background-color: var(--bg-primary); border-color: var(--border);\"><span class=\"font-semibold text-sm\" style=\"color: var(--text-primary)\">Static</span><p class=\"text-xs mt-1\" style=\"color: var(--text-secondary)\">Pick items by hand</p></div></label> <label class=\"relative cursor-pointer\"><input type=\"radio\" name=\"collection_type\" value=\"smart\" @change=\"setCollectionType($event.target.value)\" class=\"peer sr-only\"><div class=\"p-3 rounded-lg border text-center peer-checked:border-blue-500 peer-checked:ring-2 peer-checked:ring-blue-400\" style=\"background-color: var(--bg-primary); border-color: var(--border);\"><span class=\"font-semibold text-sm\" style=\"color: var(--text-primary)\">⚡ Smart</span><p class=\"text-xs mt-1\" style=\"color: var(--text-secondary)\">Match by rules — always live</p></div></label></div></div><div class=\"mb-4\" x-show=\"collectionType === 'smart'\" x-cloak><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Rules scope</label><div class=\"grid grid-cols-2 gap-2\"><button type=\"button\" @click=\"setSmartScope('books')\" :class=\"smartScope === 'books' ? 'btn btn-secondary' : 'btn btn-ghost'\">📚 Books</button> <button type=\"button\" @click=\"setSmartScope('episodes')\" :class=\"smartScope === 'episodes' ? 'btn btn-secondary' : 'btn btn-ghost'\">🎧 Podcast episodes</button></div><p class=\"text-xs mt-2\" style=\"color: var(--text-secondary)\">Episodes collections match your podcast subscriptions' episodes (feed, downloaded, played, published date) and play as a sequence.</p><input type=\"hidden\" name=\"filter_tree\" id=\"collection-tree-seed\" value=\"\"></div><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Name</label> <input type=\"text\" name=\"name\" required class=\"input\" placeholder=\"My Reading List\"></div><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Description</label> <textarea name=\"description\" class=\"input\" placeholder=\"Optional description\" rows=\"3\"></textarea></div><div class=\"mb-4\"><label class=\"block text-xs font-semibold uppercase tracking-wide mb-2\" style=\"color: var(--text-secondary)\">Icon</label> <input type=\"text\" id=\"icon-search\" class=\"input mb-2\" placeholder=\"Search or type emoji...\" maxlength=\"4\" @input=\"filterIcons($event.target.value)\" @focus=\"showAllIcons()\"> <input type=\"hidden\" name=\"icon\" id=\"collection-icon\" value=\"")
 			if templ_7745c5c3_Err != nil {
 				return templ_7745c5c3_Err
 			}
 			var templ_7745c5c3_Var8 string
 			templ_7745c5c3_Var8, templ_7745c5c3_Err = templ.ResolveAttributeValue(collection.Icon)
 			if templ_7745c5c3_Err != nil {
-				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/collection_modal.templ`, Line: 147, Col: 83}
+				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/collection_modal.templ`, Line: 170, Col: 83}
 			}
 			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var8)
 			if templ_7745c5c3_Err != nil {