Add built-in auto-updater with tab-bar update pill

Wires electrobun's Updater to the /dl download host. The packaged app checks
https://miniscp.com/dl on launch + every 6h; when a newer build exists, an
accent-colored "update tool" appears next to Connections in the tab bar. It
announces the update, becomes its own inline progress bar while downloading,
then flips to "Restart to apply" (two-step: download, then user-triggered
relaunch).

- electrobun.config.ts: release.baseUrl = https://miniscp.com/dl (baked into
  version.json at build; miniscp.com fronted by an external proxy for TLS).
- rpc.ts: checkForUpdate/downloadUpdate/applyUpdate requests + updateStatus push.
- rpc-handlers.ts + index.ts: call Updater; poll on launch+6h; stream
  onStatusChange progress. checkForUpdate self-guards the dev channel.
- rpc.service.ts + new updater.service.ts: signal-based state
  (available -> downloading -> ready, ready derived from downloadUpdate ok).
- tab-bar.ts: single bg-primary pill; progress fills the pill itself.
- icons.ts: register mynaDownload. i18n: tabBar + menu keys across all locales.
- menu.ts + app.ts: "Check for Updates…" app-menu item -> UpdaterService.check().
- docs: build-and-release + architecture updated.

Unsigned builds still update (applyUpdate strips the macOS quarantine xattr).
Verified: tsc --noEmit (exit 0) + build:ui AOT (exit 0). Live pill + full
download/apply flow require a two-build test on the host.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/architecture.md

@@ -226,6 +226,14 @@ would float over editor windows).
   `EDITOR_CONTEXT`, `IS_MAC_CHROME`, `IS_WINDOWS`, `MOD_LABEL`/`ALT_LABEL`)
   + `blockNativeContextMenu` (kills WebKit's Reload/Inspect menu outside
   text fields).
+- `updater.service.ts` — auto-updater state for the tab-bar pill. Signals
+  `state` (`idle→available→downloading→ready`|`error`), `version`, `progress`.
+  Subscribes the `updateStatus` push (backend `Updater.checkForUpdate` on
+  launch+6h, `onStatusChange` progress); `ready` is derived when the
+  `downloadUpdate` request resolves ok (not a pushed phase). `download()` /
+  `apply()` (relaunch) / `check()` (menu). Backend wiring lives in
+  `src/bun/index.ts`; feed is `release.baseUrl` (`https://miniscp.com/dl`, the
+  `/dl` host in `site/`). See build-and-release.md "Auto-updater".
 
 ## Theming