feat: add public share page with download support (#253)

Share links now point to /s/{token} (was /api/s/{token}) and render a
proper HTML page instead of raw JSON.

Changes:
- static/share.html: standalone public share page
- static/css/views/share-public.css: share page styles
- static/js/views/public/publicShare.js: client-side logic that fetches
  share metadata via /api/s/{token}, handles password-protected shares,
  and renders file download / folder info
- build.rs: include share.html in the HTML embed pipeline
- web/mod.rs: serve /s/{token} route (unauthenticated)
- share_dto.rs: generate URLs as /s/{token} instead of /api/s/{token}
- share_handler.rs: new download_shared_file() handler that validates the
  share token and streams file content without requiring authentication
- routes.rs: mount GET /api/s/{token}/download (public, uses AppState)

src/interfaces/api/routes.rs

@@ -51,6 +51,12 @@ pub fn create_public_api_routes(app_state: &Arc<AppState>) -> Router<Arc<AppStat
             .with_state(share_service);
 
         router = router.nest("/s", public_share_router);
+
+        // Download endpoint uses full AppState (needs FileRetrievalService)
+        router = router.route(
+            "/s/{token}/download",
+            get(share_handler::download_shared_file),
+        );
     }
 
     // i18n routes — no auth required (localization should be available before login)