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  matching.go MatchingHandler twins), superseded helpers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.

web/src/audio.ts

@@ -60,12 +60,12 @@ type AudioChapter = { index: number; title: string; start: number; end: number }
 // layouts) carry several; single-file books carry exactly one spanning
 // the whole file. start/end are GLOBAL seconds — the same coordinate
 // system as chapters, bookmarks and audio_offset_seconds.
-export type AudioTrack = { url: string; start: number; end: number; title?: string };
+type AudioTrack = { url: string; start: number; end: number; title?: string };
 
 // trackIndexFor finds the track holding a global position: the last one
 // whose start is at or before it (zero-duration/unknown-end tracks keep
 // working because only start matters for membership).
-export function trackIndexFor(tracks: AudioTrack[], globalSeconds: number): number {
+function trackIndexFor(tracks: AudioTrack[], globalSeconds: number): number {
   for (let i = tracks.length - 1; i > 0; i -= 1) {
     if (globalSeconds >= tracks[i].start) return i;
   }
@@ -73,7 +73,7 @@ export function trackIndexFor(tracks: AudioTrack[], globalSeconds: number): numb
 }
 
 // localOffsetFor converts a global position to seconds within track i.
-export function localOffsetFor(tracks: AudioTrack[], index: number, globalSeconds: number): number {
+function localOffsetFor(tracks: AudioTrack[], index: number, globalSeconds: number): number {
   const track = tracks[index];
   if (!track) return Math.max(0, globalSeconds);
   return Math.max(0, globalSeconds - track.start);
@@ -81,11 +81,7 @@ export function localOffsetFor(tracks: AudioTrack[], index: number, globalSecond
 
 // globalSecondsFor converts (track i, local seconds) back to the global
 // position — the inverse of localOffsetFor.
-export function globalSecondsFor(
-  tracks: AudioTrack[],
-  index: number,
-  localSeconds: number,
-): number {
+function globalSecondsFor(tracks: AudioTrack[], index: number, localSeconds: number): number {
   const track = tracks[index];
   return (track ? track.start : 0) + Math.max(0, localSeconds);
 }
@@ -1434,8 +1430,4 @@ Alpine.data("audioReader", () => ({
       this.pushProgress();
     }, PROGRESS_INTERVAL_MS);
   },
-
-  fmtTime,
 }));
-
-export { fmtTime };