feat(stats): Library tab — per-library inventory stats + audio-aware Reading tab

The /stats page gains a second tab (Reading | Library) implementing the
Audiobookshelf-style library dashboard (HANDOFF item 9): media_items
INVENTORY aggregates — items, hours/pages, distinct authors, size,
distinct narrators, Top Genres / Top Authors bars, Longest / Largest
items — for one library or the All-libraries aggregate.

- GET /api/stats/library-usage?library_id=<uuid|omit>; family lens
  enforced: a library_id outside the requester's visible set is a 403,
  never a silent pass (AGENTS.md standing rule; 431fec0 history)
- six sqlc queries with the visibility pattern + healthy-item filter
  (archived_at IS NULL AND missing_scan_count = 0); hours_mode lets the
  client swap the Hours/Pages card and the Longest ranking per scope
- Reading tab unit audit (HANDOFF item 3, settled 2026-10-16): minutes
  are the merged currency everywhere honest; only the PAGES unit splits
  — Pages card goes mode-aware (Pages · text / Listening Time in an
  audio-only pool) and the charts row gains Listening per Day (hrs)
  when audio minutes exist in the range; summary daily buckets gain
  audio_minutes split by the item's library type
- tab state in the URL (?tab=library&library_id=…); selector fed by the
  existing /api/libraries/visibility endpoint; main.js?v=4
- integration tests: family-lens 403 pin, hidden-library exclusion
  (differential), seeded text+audio aggregate correctness, auth gate
- docs/developer/api/stats/get_library_usage.md

Known-remaining suite failures are environmental: TestConflictList_Empty
+ TestWebSocketPingPong collide with the concurrently-running app session
(live emulator pushes + scans on the shared dev DB), and
TestAdminCreateUser's timezone expectation drifted from live system
settings — all three fail on the clean tree too.

templates/header_templ.go

@@ -631,7 +631,7 @@ func Header(user User, currentPath string) templ.Component {
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 64, "</span> <input type=\"text\" id=\"header-search\" placeholder=\"Search your library…\" class=\"input pl-10\" autocomplete=\"off\"></div></div></header><!-- module bundle; ?v=<commit-ish stamp> busts stale intermediate\n\t\t     builds across app restarts (browser caches are keyed by URL) --><script type=\"module\" src=\"/static/main.js?v=3\"></script></div>")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 64, "</span> <input type=\"text\" id=\"header-search\" placeholder=\"Search your library…\" class=\"input pl-10\" autocomplete=\"off\"></div></div></header><!-- module bundle; ?v=<commit-ish stamp> busts stale intermediate\n\t\t     builds across app restarts (browser caches are keyed by URL) --><script type=\"module\" src=\"/static/main.js?v=4\"></script></div>")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}