feat: auto-retry uploads when rate limited (v0.13.0)

Backend: rate limiter gains check_with_retry() returning seconds until
the next slot opens. Upload 429 responses include retry_after_secs in
JSON and a Retry-After header.

Frontend: upload queue catches 429 as RateLimitError, resets affected
item to pending, schedules processQueue() for the server-reported delay,
and shows a live countdown banner in the queue UI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/services/rate_limiter.rs

@@ -19,17 +19,26 @@ impl RateLimiter {
 
     /// Returns `true` if the request is allowed, `false` if rate-limited.
     pub fn check(&self, key: impl Into<String>, max: usize, window: Duration) -> bool {
+        self.check_with_retry(key, max, window).is_ok()
+    }
+
+    /// Returns `Ok(())` if allowed, `Err(retry_after_secs)` if rate-limited.
+    /// `retry_after_secs` is how long until the oldest slot in the window expires.
+    pub fn check_with_retry(&self, key: impl Into<String>, max: usize, window: Duration) -> Result<(), u64> {
         let now = Instant::now();
         let key = key.into();
         let mut map = self.windows.lock().unwrap();
         let timestamps = map.entry(key).or_default();
-        // Drop entries outside the window
         timestamps.retain(|&t| now.duration_since(t) < window);
         if timestamps.len() < max {
             timestamps.push(now);
-            true
+            Ok(())
         } else {
-            false
+            // The oldest timestamp expires at oldest + window; compute remaining seconds
+            let oldest = timestamps[0];
+            let elapsed = now.duration_since(oldest);
+            let remaining = window.saturating_sub(elapsed);
+            Err(remaining.as_secs().max(1))
         }
     }
 }