fix(desktop): give Windows a title bar and report every update stage (#151)

## Related Issue

No issue — reported directly: on Windows the caption buttons cover the
git branch and status in the conversation header, the window has no
title bar, and the app never says a new version is available.

## Problem

Three defects, all in the desktop shell:

1. **Windows has no title bar.** The desktop shell drops
`titleBarOverlay` on win32 and the renderer owns the caption, but it
drew the buttons as a `position: fixed` overlay pinned to the top-right
— directly on top of the conversation header, covering the git branch
and diff counts. The window also had no drag region at all on Windows:
the two that exist (`Sidebar.vue`, `ChatHeader.vue`) are both gated on
`macos-desktop`, so the window could not be moved.
2. **The update prompt only appears at the very last step.** Automatic
downloads are the default, and `UpdateToast` rendered only for
`downloaded` (or `available` with automatic downloads off). So "a new
version was found", "it is downloading", and "the download failed" were
unobservable — and there is no other update surface anywhere in the
product: no tray item, no settings toggle, no menu. A user whose
download fails is told nothing, ever.
3. **The "Internal testing only" tag** shows in every desktop build, on
macOS and Windows alike.

## What changed

**Windows title bar.** `WindowControls.vue` is now an in-flow 32px strip
above the app instead of an overlay. `.app-shell` is already a flex
column, so `.app` simply shrinks and nothing can sit under the buttons.
The strip is the window-drag region, with the buttons opting out — that
is what restores dragging on Windows. `window-options.ts` is unchanged:
`titleBarStyle: 'hidden'` + `thickFrame` is already the correct shell
for a renderer-drawn caption.

**Update prompt.** `UpdateToast.vue` now reports found → downloading
(with percent) → ready to install → failed, with the install action
enabled only in the two states that can start one. A failure is surfaced
only once an update was actually found in this session, so a check that
fails on an offline machine stays quiet; dismissing a failure no longer
writes to the skipped-versions list, so the restart prompt still arrives
when the retry succeeds.

**Internal-testing tag** removed, along with its locale key and layout
rule.

All three are gated on the desktop shell, so CLI and browser users
perceive no change — the changesets name `@pymodel/pythinker-desktop`,
not the CLI. The committed `dist-web` bundle is restaged in the same
commit, since the desktop app loads that bundle rather than
`apps/pythinker-web/src`.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] I have linked a related issue, or explained the problem above.
- [x] I have added tests that prove my feature works. —
`update-toast.test.ts` covers each state, the retry/dismiss split, and
the offline-quiet rule; `window-controls.test.ts` asserts the bar is
in-flow and drags. 913 tests pass in `apps/pythinker-web`.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset. — three
entries under `.changeset/`, minor for the two fixes and patch for the
tag removal.
- [x] Ran `gen-docs` skill, or this PR needs no doc update. — no
documented CLI behavior changes.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added clearer update notifications for available, downloading,
downloaded, and failed updates, with retry and dismiss actions.
  * Added progress and restart guidance for desktop updates.
  * Improved the Windows title bar layout and window-drag behavior.

* **Bug Fixes**
* Update controls now remain correctly enabled or disabled based on
update status.

* **Style**
  * Removed the “Internal testing only” banner.
  * Long update messages now wrap correctly.

* **Chores**
  * Refreshed bundled web assets and editor language support.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/drop-internal-build-tag.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-desktop": patch
+---
+
+Remove the "Internal testing only" tag.

.changeset/update-prompt-states.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-desktop": minor
+---
+
+Say when a new version is found, while it downloads, and when a download fails, instead of only once it is ready to install.

.changeset/windows-title-bar.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-desktop": minor
+---
+
+Give the Windows window its own title bar, so the window controls no longer sit on top of the conversation header and the window can be dragged again.

apps/pythinker-code/dist-web/.web-bundle-manifest.json

@@ -1,4 +1,4 @@
 {
-  "sourceHash": "39adcc1ccb551ac03f06fe2152c6a69004701da14fd2535f8b26202cb9d27c14",
-  "sourceFileCount": 389
+  "sourceHash": "5fa1f4113f68cc5a0de3b4985ae517b47ca98f832e89e97cf636845fd3a2cd67",
+  "sourceFileCount": 388
 }