feat(dashboard): collapsible This Week widget, library-scoped + switch-surviving; title into the subbar

Three dashboard complaints, one build:

- The widget collapses to a single summary row (This Week · minutes ·
  pages · sessions · books · streak · Full stats) on every screen size
  and expands to the full grid via a details/summary toggle — the
  carousel sections start immediately below it again.
- The 'Your Library' heading moved into the library switcher subbar
  (LibrarySwitcher gains an optional title param; collections/series
  keep the plain 'Library' label). The standalone heading block is
  gone.
- Root-caused the JS-switch vanishing: renderDashboardCollections
  replaces ALL of #collections-container's innerHTML, so anything
  server-rendered inside it (title, widget) died on the first library
  switch. The widget now re-fetches ITSELF: a /dashboard/week-widget
  fragment (same resolveLibrary family-lens rules as the page) is
  pulled by htmx whenever the switcher dispatches week-refresh —
  carrying the current #library-select value — so the widget both
  survives switches and RE-SCOPES to the switched library. Initial SSR
  renders the same scope the sections use.

ThisWeek (and the streak calendar) gained an optional library filter.
Tests: TestReadingStatsWeekLibraryScope — per-library totals, scoped
streaks, invisible-library isolation, malformed id 400. Full suite
PASS.

templates/library_switcher.templ

@@ -1,13 +1,17 @@
 package templates
 
-templ LibrarySwitcher(libData []LibraryData, currentLibraryID string, actions ...templ.Component) {
+templ LibrarySwitcher(libData []LibraryData, currentLibraryID string, title string, actions ...templ.Component) {
 	<div
 		class="app-subbar backdrop-blur border-b"
 		style="background-color: color-mix(in srgb, var(--bg-primary) 88%, transparent); border-color: color-mix(in srgb, var(--text-primary) 6%, transparent);"
 	>
 		<div class="w-full px-4 sm:px-6 lg:px-8 py-3 flex items-center justify-between gap-4">
-			<div class="flex items-center gap-3">
-				<label class="text-sm font-medium" style="color: var(--text-secondary)">Library</label>
+			<div class="flex items-center gap-3 min-w-0">
+				if title != "" {
+					<span class="text-base font-bold tracking-tight shrink-0" style="color: var(--text-primary)">{ title }</span>
+				} else {
+					<label class="text-sm font-medium" style="color: var(--text-secondary)">Library</label>
+				}
 				<select
 					id="library-select"
 					name="library_id"