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/web/mod.rs

@@ -59,6 +59,7 @@ pub fn create_web_routes() -> Router<Arc<AppState>> {
         .route("/profile", get(serve_profile_page))
         .route("/admin", get(serve_admin_page))
         .route("/device", get(serve_device_verify_page))
+        .route("/s/{token}", get(serve_share_page))
         // Serve static files with compression + cache headers
         .fallback_service(static_service)
         .layer(CompressionLayer::new().br(true).gzip(true))
@@ -90,3 +91,8 @@ async fn serve_device_verify_page() -> Html<&'static str> {
         "/device-verify.html"
     )))
 }
+
+/// Serve the public share page (unauthenticated)
+async fn serve_share_page() -> Html<&'static str> {
+    Html(include_str!(concat!(env!("OUT_DIR"), "/share.html")))
+}