feat(menubar): manual "Check for updates" in the About box (T-47 P1)

Help → About gains a "Check for updates" button that fetches the latest GitHub
release, semver-compares it to clideVersion, and shows the result inline:
up-to-date, available (with a tappable link to the release notes), or a clear
error. clide's first and only outbound HTTP call — a plain GET with no user
data, run ONLY on this explicit tap, never on a launch path or a timer. So it's
D-64-clean with no amendment; a background/periodic poll stays deferred (would
need the narrow opt-in amendment first).

The fetch is injectable so no test touches the network. compareSemver handles
2.3.10 > 2.3.9 and ranks pre-releases below their release. Closes T-492 (P1);
the release-channel CI for downloadable signed packages is T-491, and download/
apply (P2/P3) depend on it.

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

assets/i18n/en_us/builtin.menubar.json

@@ -6,6 +6,11 @@
   "about.commit": { "translation": "Commit" },
   "about.built": { "translation": "Built" },
   "about.repository": { "translation": "Repository" },
+  "about.checkUpdates": { "translation": "Check for updates" },
+  "about.checking": { "translation": "Checking…" },
+  "about.upToDate": { "translation": "You're on the latest version." },
+  "about.updateAvailable": { "translation": "clide {version} is available — release notes" },
+  "about.updateFailed": { "translation": "Couldn't check for updates" },
   "licenses.heading": { "translation": "Bundled dependencies" },
   "licenses.unavailable": { "translation": "Licenses unavailable." },
   "licenses.loading": { "translation": "Loading…" },

assets/i18n/nl_nl/builtin.menubar.json

@@ -6,6 +6,11 @@
   "about.commit": { "translation": "Commit" },
   "about.built": { "translation": "Gebouwd" },
   "about.repository": { "translation": "Repository" },
+  "about.checkUpdates": { "translation": "Controleer op updates" },
+  "about.checking": { "translation": "Bezig met controleren…" },
+  "about.upToDate": { "translation": "Je hebt de nieuwste versie." },
+  "about.updateAvailable": { "translation": "clide {version} is beschikbaar — releaseopmerkingen" },
+  "about.updateFailed": { "translation": "Kon niet op updates controleren" },
   "licenses.heading": { "translation": "Meegeleverde afhankelijkheden" },
   "licenses.unavailable": { "translation": "Licenties niet beschikbaar." },
   "licenses.loading": { "translation": "Laden…" },