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  matching.go MatchingHandler twins), superseded helpers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/services/collection_service.go

@@ -122,27 +122,6 @@ func (s *CollectionService) GetCollection(ctx context.Context, collectionID uuid
 	return collection, nil
 }
 
-// GetCollectionWithBookCount gets a collection with book count
-func (s *CollectionService) GetCollectionWithBookCount(ctx context.Context, collectionID uuid.UUID) (map[string]any, error) {
-	collection, err := s.db.GetCollectionWithBookCount(ctx, pgtype.UUID{Bytes: collectionID, Valid: true})
-	if err != nil {
-		return nil, fmt.Errorf("failed to get collection: %v", err)
-	}
-
-	return map[string]any{
-		"id":                uuid.UUID(collection.ID.Bytes).String(),
-		"user_id":           uuid.UUID(collection.UserID.Bytes).String(),
-		"name":              collection.Name,
-		"description":       collection.Description.String,
-		"color":             collection.Color.String,
-		"icon":              collection.Icon.String,
-		"auto_assign_rules": collection.AutoAssignRules,
-		"view_settings":     collection.ViewSettings,
-		"created_at":        collection.CreatedAt.Time.String(),
-		"book_count":        collection.BookCount,
-	}, nil
-}
-
 // GetUserCollections gets all collections for a user
 func (s *CollectionService) GetUserCollections(ctx context.Context, userID uuid.UUID) ([]database.Collections, error) {
 	collections, err := s.db.GetCollectionsByUser(ctx, pgtype.UUID{Bytes: userID, Valid: true})