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>

neode-ui/src/views/settings/AccountInfoSection.vue

@@ -180,6 +180,18 @@ init()
               </button>
             </div>
             <div class="overflow-y-auto flex-1 min-h-0 space-y-6 pr-1">
+              <!-- v1.7.17-alpha -->
+              <div>
+                <div class="flex items-center gap-2 mb-3">
+                  <span class="text-xs font-mono px-2 py-0.5 rounded bg-orange-500/20 text-orange-300">v1.7.17-alpha</span>
+                  <span class="text-xs text-white/40">Apr 20, 2026</span>
+                </div>
+                <div class="space-y-3 text-sm text-white/80 pl-3 border-l border-white/10">
+                  <p>When a download gets stuck, you can now cancel it. A new Cancel Download button sits next to the progress bar — it stops the transfer, clears the partial file, and returns you to a clean state so you can retry. No more staring at a frozen bar with no way to recover.</p>
+                  <p>Downloads that stall for 30 seconds or more now say so. The progress bar turns amber and shows 'Download appears stuck — try Cancel and start again' instead of just sitting silently at whatever percent it reached.</p>
+                  <p>Canceling is fast. It no longer has to wait out the retry timer — the download bails within half a second, so you're not stuck watching a stuck screen while you wait to unstick it.</p>
+                </div>
+              </div>
               <!-- v1.7.16-alpha -->
               <div>
                 <div class="flex items-center gap-2 mb-3">