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  matching.go MatchingHandler twins), superseded helpers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/handlers/library.go

@@ -323,16 +323,6 @@ func parseUUID(uuidStr string) (pgtype.UUID, error) {
 	return pgtype.UUID{Bytes: parsedUUID, Valid: true}, nil
 }
 
-// GetUserVisibleLibrariesData returns libraries for SSR (not JSON response)
-func (h *LibraryHandler) GetUserVisibleLibrariesData(ctx context.Context, userID pgtype.UUID) ([]database.GetUserVisibleLibrariesRow, error) {
-	return h.libraryService.GetUserVisibleLibraries(ctx, userID)
-}
-
-// GetLibraryTypeData returns all library types for SSR (not JSON response)
-func (h *LibraryHandler) GetLibraryTypeData(ctx context.Context) ([]database.LibraryTypes, error) {
-	return h.libraryService.GetLibraryTypes(ctx)
-}
-
 // ListLibrariesData returns all libraries for SSR (admin only, not JSON response)
 func (h *LibraryHandler) ListLibrariesData(ctx context.Context) ([]database.ListLibrariesRow, error) {
 	return h.libraryService.ListLibraries(ctx)