release(v1.7.27-alpha): mirror transparency — served-by line + one-click test button

- New "Served by {mirror}" line on the System Update page so operators can see
  which mirror actually served the available manifest (vs. which is configured
  primary). Backend threads the served URL through UpdateState.manifest_mirror.
- New update.test-mirror RPC + per-row lightning-bolt button that pings a
  mirror and renders reachable/latency or error inline under the URL.
- UI polish on the mirrors section: Set Primary, Remove, and the new Test
  action are compact icon buttons; add-mirror form moved into a dialog.
- "What's New" block prepended for v1.7.27-alpha.

21/21 update module tests pass. vue-tsc + vite build clean.

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">
+              <!-- v1.7.27-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">v1.7.27-alpha</span>
+                  <span class="text-xs text-white/40">Apr 21, 2026</span>
+                </div>
+                <div class="space-y-3 text-sm text-white/80 pl-3 border-l border-white/10">
+                  <p>The Update page now shows which mirror delivered your update — a small "Served by" line under the new version tells you whether Server 1, Server 2, or a custom mirror was the one your node actually reached. Great for spot-checking that mirror fallback is doing its job.</p>
+                  <p>Every mirror row has a new lightning-bolt button that pings the mirror and shows whether it's reachable, plus the round-trip latency in milliseconds. No more guessing if a mirror you just added is responding.</p>
+                  <p>The Update mirrors section got a visual refresh: Set Primary, Remove, and the new Test action are compact icon buttons instead of crowded text, and adding a mirror now happens in a dedicated dialog that matches the rest of the UI.</p>
+                </div>
+              </div>
               <!-- v1.7.26-alpha -->
               <div>
                 <div class="flex items-center gap-2 mb-3">