refactor: simplify PWA handlers and fix review findings

Address code review findings from reuse, quality, and efficiency agents:

- Cache manifest JSON and service worker JS at init (was rebuilt per
  request with allocations and JSON encoding on every hit)
- Add ImagePathsByUser store method for targeted image cleanup (was
  loading 100k full fave objects just to read image_path)
- Add missing aria-label on privacy toggle in fave_list.html (inline
  copy had drifted from the partial — accessibility bug)
- Fix comment/function name mismatch in pwa.go
- Remove redundant user nil-check in handleShare (requireLogin guards)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/store/fave.go

@@ -97,6 +97,29 @@ func (s *FaveStore) Delete(id int64) error {
 	return nil
 }
 
+// ImagePathsByUser returns all non-empty image paths for a user's faves.
+// Used for cleanup before user deletion.
+func (s *FaveStore) ImagePathsByUser(userID int64) ([]string, error) {
+	rows, err := s.db.Query(
+		"SELECT image_path FROM faves WHERE user_id = ? AND image_path != ''",
+		userID,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var paths []string
+	for rows.Next() {
+		var p string
+		if err := rows.Scan(&p); err != nil {
+			return nil, err
+		}
+		paths = append(paths, p)
+	}
+	return paths, rows.Err()
+}
+
 // ListByUser returns all faves for a user (both public and private),
 // ordered by newest first, with pagination.
 func (s *FaveStore) ListByUser(userID int64, limit, offset int) ([]*model.Fave, int, error) {