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/application/dtos/share_dto.rs

@@ -46,7 +46,7 @@ pub struct UpdateShareDto {
 /// Extension methods to convert between DTOs and domain entities
 impl ShareDto {
     pub fn from_entity(share: &Share, base_url: &str) -> Self {
-        let url = format!("{}/api/s/{}", base_url, share.token());
+        let url = format!("{}/s/{}", base_url, share.token());
 
         Self {
             id: share.id().to_string(),

src/application/services/share_service.rs

@@ -1135,6 +1135,6 @@ mod tests {
         assert_eq!(share_dto.item_id, "test_file_id");
         assert_eq!(share_dto.item_type, "file");
         assert!(share_dto.has_password);
-        assert!(share_dto.url.starts_with("http://127.0.0.1:8086/api/s/"));
+        assert!(share_dto.url.starts_with("http://127.0.0.1:8086/s/"));
     }
 }