feat(extension): in-app update prompt — popup banner + toolbar badge (#1489)

The extension is installed per-instance from the operator's FC host, so Firefox's
static update_url can't apply (each instance has a different host) and updates
were fully manual. Add a self-hosted-friendly update surface that reuses the
existing public GET /api/extension/manifest ({version, latest_url, sha256}):

- lib/api.js: getExtensionManifest().
- background.js: checkForUpdateInfo() compares the instance's latest published
  version against runtime.getManifest().version (dotted-numeric compare so
  1.0.10 > 1.0.9); CHECK_UPDATE message handler; refreshUpdateBadge() sets a
  toolbar badge via browser.action; a daily browser.alarms check plus on
  startup/installed. New 'alarms' permission (non-prompting).
- popup: an 'Update available — vX' banner with an Update button that opens the
  signed XPI (web root, /api stripped like OPEN_ARTIST_PAGE) → Firefox's native
  install prompt. Never blocks the popup on a failed check.

No backend changes (endpoint already exists). Bump 1.0.8→1.0.9 so this ships;
from here on updates surface themselves instead of needing a manual reinstall.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

extension/popup/popup.css

@@ -72,6 +72,17 @@ body {
 .btn.block { display: block; width: 100%; margin-top: 8px; }
 .btn.link { background: none; color: var(--on-surface-variant); padding: 4px; }
 .btn.link:hover { color: var(--accent); }
+.btn.small { padding: 6px 12px; font-size: 13px; }
+
+/* In-app update prompt (accent-tinted so it reads as an actionable notice). */
+.update-banner {
+  display: flex; align-items: center; gap: 10px;
+  margin: 10px 10px 0; padding: 10px 12px;
+  background: rgba(244, 186, 122, 0.12);
+  border: 1px solid rgba(244, 186, 122, 0.4);
+  border-radius: 6px;
+}
+#update-text { flex: 1; font-size: 13px; }
 
 .source-row .play {
   background: none; border: none; color: var(--on-surface-variant);