refactor: remove dead code across server, templ pages, and client modules

Deadcode-verified removals (RTA reachability from main, cross-checked
with -test and textual xrefs):

Go server:
- whole files: middleware/transaction.go, middleware/error_handler.go,
  middleware/password_validator.go (live registration machinery kept),
  handlers/sync.go, sync/offline.go
- unrouted endpoints: auth UpdateUsername/UpdateEmail, timezone
  settings handler, OPDS RegisterOPDS, Kobo LibrarySync handler,
  ReaderHandler.ShowReader, JSON-data twins on collections/library
- duplicate book-matching handlers (book_matching.go Handler twins,
  matching.go MatchingHandler twins), superseded helpers
  (router.parseUUID, handlers.ValidateDeviceToken, normalizePath,
  userExists, generateLockID)

Templ/client:
- never-routed pages: api_explorer.templ, queue.templ and their
  client modules api-explorer.ts, api-explorer-docs.ts, queue.ts
- fully orphaned wrapper modules events.ts, dom.ts
- broken clearSyncQueue (called a nonexistent endpoint) and the
  hidden, never-populated sync-queue section on the devices page
- 166+ unused exports demoted to private (knip-verified); ~85 truly
  dead declarations removed via tsc noUnusedLocals fixpoint
- stale 'interactive API Explorer' claims dropped from docs

The docs drift-gate package (internal/docs), templ-bound modules
(dashboard, stats, header, index, login, profile*, bookshelf,
collections, toast-error), sqlc-generated code, and web/static are
intentionally untouched.

internal/services/media_scanner_audio.go

@@ -476,20 +476,6 @@ func applyAudioTagFallbacks(metadata *MediaMetadata, report *ffprobeReport) {
 	}
 }
 
-// audioDurationLabel renders the duration for logs/detail surfaces.
-func audioDurationLabel(seconds float64) string {
-	if seconds <= 0 {
-		return "—"
-	}
-	d := time.Duration(seconds) * time.Second
-	h := int(d.Hours())
-	m := int(d.Minutes()) % 60
-	if h > 0 {
-		return fmt.Sprintf("%dh %dm", h, m)
-	}
-	return fmt.Sprintf("%dm", m)
-}
-
 // parseJSONInt reads a RawMessage that may hold a JSON number or a
 // number-string; 0 for anything else. Used for sidecar fields whose
 // type differs across producing tools (Libation vs Calibre/ABS).