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  matching.go MatchingHandler twins), superseded helpers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.

templates/devices.templ

@@ -183,15 +183,8 @@ templ Devices(user User, devices []handlers.DeviceInfo, pendingRegistrations []P
 						</div>
 					</div>
 				}
-				<div id="sync-queue-section" class="mt-12 hidden">
-					<div class="flex justify-between items-center mb-4">
-						<h2 class="text-lg font-bold tracking-tight" style="color: var(--text-primary)">Sync Queue</h2>
-						<button @click="clearSyncQueue()" class="btn btn-secondary">Clear Queue</button>
-					</div>
-					<div id="sync-queue" class="space-y-3"></div>
-				</div>
 			</div>
-		<div id="add-device-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center p-4" x-data="{ selectedType: '' }" style="background-color: var(--surface-overlay);">
+				<div id="add-device-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center p-4" x-data="{ selectedType: '' }" style="background-color: var(--surface-overlay);">
 			<div class="card p-6 w-full max-w-md" style="box-shadow: var(--shadow-pop);">
 				<div class="flex justify-between items-center mb-6">
 					<h2 class="text-xl font-bold" style="color: var(--text-primary)">Add New Device</h2>