release(v1.7.17-alpha): cancel download + stall detection

Add Cancel Download button + stall detection so a wedged download can
be recovered instead of leaving the UI stuck on a frozen progress bar.

Backend:
- update.rs: DOWNLOAD_CANCEL AtomicBool + DOWNLOAD_PROGRESS_AT AtomicU64
- download loop checks cancel between chunks and during retry backoff
  (500ms slices instead of one exponential sleep, so Cancel wakes fast)
- cancel_download() wipes staging + clears update_in_progress
- update.status exposes download_progress.stalled (30s no-progress)
- RPC: update.cancel-download + dispatcher entry

Frontend:
- SystemUpdate.vue: Cancel Download button, amber stall styling,
  stalled copy, cancel-download confirm branch in modal
- i18n keys (en + es) for cancel/stall flow
- v1.7.17-alpha What's New block in AccountInfoSection

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/update.rs

@@ -168,6 +168,27 @@ impl RpcHandler {
         let active = total > 0 && downloaded < total;
         let completed = total > 0 && downloaded >= total;
 
+        // Stall detection: if the progress-at timestamp hasn't advanced
+        // for 30+ seconds while active, the download is wedged (usually
+        // HTTP stream silently dropped and reqwest is waiting out its
+        // read timeout). The UI uses this to surface a Cancel button
+        // with explanatory copy.
+        let stalled = if active {
+            let last_at = update::DOWNLOAD_PROGRESS_AT
+                .load(std::sync::atomic::Ordering::Relaxed);
+            if last_at > 0 {
+                let now = std::time::SystemTime::now()
+                    .duration_since(std::time::UNIX_EPOCH)
+                    .map(|d| d.as_millis() as u64)
+                    .unwrap_or(0);
+                now.saturating_sub(last_at) > 30_000
+            } else {
+                false
+            }
+        } else {
+            false
+        };
+
         Ok(serde_json::json!({
             "current_version": state.current_version,
             "last_check": state.last_check,
@@ -179,6 +200,7 @@ impl RpcHandler {
                     "bytes_downloaded": downloaded,
                     "total_bytes": total,
                     "active": active,
+                    "stalled": stalled,
                 }))
             } else { None },
         }))
@@ -200,6 +222,13 @@ impl RpcHandler {
         }))
     }
 
+    /// Cancel an in-flight or stuck download. Clears the live counters
+    /// and staging dir so the UI returns to the "Download Update" state.
+    pub(super) async fn handle_update_cancel_download(&self) -> Result<serde_json::Value> {
+        update::cancel_download(&self.config.data_dir).await?;
+        Ok(serde_json::json!({ "canceled": true }))
+    }
+
     /// Apply the staged update.
     pub(super) async fn handle_update_apply(&self) -> Result<serde_json::Value> {
         update::apply_update(&self.config.data_dir).await?;