feat: add URL validation for app namespace links

validates all app URLs concurrently on page load:
- created /api/validate-url endpoint with HEAD request and 3s timeout
- grays out invalid links with reduced opacity
- adds tooltip explaining domain is not reachable
- disables clicking on invalid links

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

src/main.rs

···
             .service(routes::get_mst)
             .service(routes::init)
             .service(routes::get_avatar)
+            .service(routes::validate_url)
             .service(routes::favicon)
             .service(Files::new("/static", "./static"))
     })

src/routes.rs

···
         "avatarUrl": null
     }))
 }
+
+#[derive(Deserialize)]
+pub struct ValidateUrlQuery {
+    url: String,
+}
+
+#[get("/api/validate-url")]
+pub async fn validate_url(query: web::Query<ValidateUrlQuery>) -> HttpResponse {
+    let url = &query.url;
+
+    // Try to make a HEAD request with a short timeout
+    let client = reqwest::Client::builder()
+        .timeout(std::time::Duration::from_secs(3))
+        .build()
+        .unwrap();
+
+    let is_valid = match client.head(url).send().await {
+        Ok(response) => response.status().is_success() || response.status().is_redirection(),
+        Err(_) => false,
+    };
+
+    HttpResponse::Ok().json(serde_json::json!({
+        "valid": is_valid
+    }))
+}

src/templates.rs

···
             color: var(--text);
         }}
 
+        .app-name.invalid-link {{
+            color: var(--text-light);
+            opacity: 0.5;
+            cursor: not-allowed;
+        }}
+
+        .app-name.invalid-link:hover {{
+            text-decoration: none;
+            color: var(--text-light);
+        }}
+
         .detail-panel {{
             position: fixed;
             top: 0;

static/app.js

···
 
             div.innerHTML = `
                 <div class="app-circle" data-namespace="${namespace}">${firstLetter}</div>
-                <a href="${url}" target="_blank" rel="noopener noreferrer" class="app-name">${displayName}</a>
+                <a href="${url}" target="_blank" rel="noopener noreferrer" class="app-name" data-url="${url}">${displayName}</a>
             `;
 
             // Try to fetch and display avatar
···
                     circle.innerHTML = `<img src="${avatarUrl}" class="app-logo" alt="${namespace}" />`;
                 }
             });
+
+            // Validate URL
+            fetch(`/api/validate-url?url=${encodeURIComponent(url)}`)
+                .then(r => r.json())
+                .then(data => {
+                    const link = div.querySelector('.app-name');
+                    if (!data.valid) {
+                        link.classList.add('invalid-link');
+                        link.setAttribute('title', 'this domain is not reachable');
+                        link.style.pointerEvents = 'none';
+                    }
+                })
+                .catch(() => {
+                    // Silently fail validation check
+                });
 
             div.addEventListener('click', () => {
                 const detail = document.getElementById('detail');