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  matching.go MatchingHandler twins), superseded helpers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/test_helpers_test.go

@@ -470,26 +470,6 @@ func (s *TestDeviceSetup) CreateLibrary(t *testing.T, name, libraryType string)
 	return libraryUUID.String()
 }
 
-// CreateCollection creates a test collection for the TestDeviceSetup
-func (s *TestDeviceSetup) CreateCollection(t *testing.T, name string) string {
-	ctx := context.Background()
-
-	pgUserID := pgtype.UUID{Bytes: [16]byte(s.User.ID), Valid: true}
-	collection, err := s.DB.CreateCollection(ctx, database.CreateCollectionParams{
-		UserID:      pgUserID,
-		Name:        name,
-		Description: pgtype.Text{String: "Test collection description", Valid: true},
-		Color:       pgtype.Text{String: "#FF5733", Valid: true},
-		Icon:        pgtype.Text{String: "folder", Valid: true},
-	})
-	require.NoError(t, err, "Should create collection")
-
-	collectionUUID, err := uuid.FromBytes(collection.ID.Bytes[0:16])
-	require.NoError(t, err, "Should parse collection ID")
-
-	return collectionUUID.String()
-}
-
 // setupTestServer creates a test server with a test database
 // Returns: *TestServerSetup with automatic cleanup via t.Cleanup
 func setupTestServer(t *testing.T) *TestServerSetup {