add ClideLightbox — click image cards to enlarge (T-252)

The inline image cards (T-249) are often too small to read. Clicking one now
opens a full-screen lightbox: zoom (scroll wheel / pinch), pan when zoomed,
double-click to reset to fit, Esc / close button / backdrop click to dismiss.

ClideLightbox is a reusable primitive (lib/widgets/) over Flutter's
InteractiveViewer with clide-owned zoom gestures, shown via the DialogRouter
(dimmed backdrop, single modal at a time, D-78). The card stays display-only;
the click is a navigation gesture, not an inline control.

CLI parity (D-6): `clide image show <path> --fullscreen` opens straight into
the lightbox instead of injecting a card.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/widgets.dart

@@ -11,6 +11,7 @@ export 'src/clide_column_hat.dart';
 export 'src/clide_code_block.dart';
 export 'src/clide_divider.dart';
 export 'src/clide_filter_box.dart';
+export 'src/clide_lightbox.dart';
 export 'src/clide_markdown.dart';
 export 'src/clide_marquee.dart';
 export 'src/clide_svg_view.dart';