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  matching.go MatchingHandler twins), superseded helpers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/reader/pdf-search.ts

@@ -6,7 +6,7 @@
 // are positional. Pages are therefore joined gap-aware, with a char→item
 // map so each match can be covered by the rects of the items it spans.
 
-export interface PdfSearchItem {
+interface PdfSearchItem {
   /** page-fraction rect of this text item */
   x: number;
   y: number;
@@ -41,7 +41,7 @@ interface RawItem {
 }
 
 /** Join one page's text items into a searchable string + item map. */
-export function buildPageText(
+function buildPageText(
   rawItems: RawItem[],
   viewportWidth: number,
   viewportHeight: number,

web/src/reader/settings-manager.ts

@@ -43,27 +43,7 @@ export async function saveSettings(settings: Partial<ReaderSettings>): Promise<v
     localStorage.setItem(LOCALSTORAGE_KEY, JSON.stringify(updated));
   }
 }
-export async function syncSettings(): Promise<void> {
-  const local = localStorage.getItem(LOCALSTORAGE_KEY);
-  if (!local) return;
-  const settings = JSON.parse(local);
-  const token = getToken();
-  if (token) {
-    try {
-      await fetch("/readers/settings", {
-        method: "PUT",
-        headers: {
-          Authorization: `Bearer ${token}`,
-          "Content-Type": "application/json",
-        },
-        body: JSON.stringify(settings),
-      });
-    } catch (error) {
-      console.error("Failed to sync settings:", error);
-    }
-  }
-}
-export function getDefaultSettings(): ReaderSettings {
+function getDefaultSettings(): ReaderSettings {
   return {
     chrome_behavior: "auto-hide",
     progress_mode: "pages",