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  matching.go MatchingHandler twins), superseded helpers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.

cmd/server/tests/ownership_and_admin_test.go

@@ -88,7 +88,7 @@ func (s *OwnershipAndAdminTestSuite) createItemViaDB(withFolder bool) (itemID, l
 	item, err := db.CreateMediaItem(ctx, database.CreateMediaItemParams{
 		LibraryID: library.ID,
 		Title:     "Ownership Suite Item",
-		Authors:  []string{"Suite Author"},
+		Authors:   []string{"Suite Author"},
 		FilePath:  relPath,
 		MimeType:  pgtype.Text{String: "application/epub+zip", Valid: true},
 	})
@@ -101,18 +101,6 @@ func (s *OwnershipAndAdminTestSuite) TearDownSuite() {
 	s.setup.Close()
 }
 
-func (s *OwnershipAndAdminTestSuite) getItem(itemID string) map[string]interface{} {
-	req, _ := http.NewRequest("GET", s.setup.Server.URL+"/api/media-items/"+itemID, nil)
-	req.Header.Set("Authorization", "Bearer "+s.token)
-	resp, err := (&http.Client{}).Do(req)
-	require.NoError(s.T(), err)
-	defer resp.Body.Close()
-	require.Equal(s.T(), http.StatusOK, resp.StatusCode)
-	var body map[string]interface{}
-	require.NoError(s.T(), json.NewDecoder(resp.Body).Decode(&body))
-	return body
-}
-
 func (s *OwnershipAndAdminTestSuite) createHighlight(token string) map[string]interface{} {
 	payload, _ := json.Marshal(map[string]interface{}{
 		"selection_text": "owned annotation text",