[Feature] 为APP添加检查新版本以及更新APP功能吧，再添加上github链接

[luchao900]

为APP添加
{
检查新版本
更新APP
github链接
}

[asdfg314284230]

deepseek update

[yuhg92]

deepseek update 命令可以更新到最新版本，不过它检查后不管是不是最新版本都会重新下载

[Hmbown]

Most of the machinery already exists — it's just not surfaced in the TUI. crates/cli/src/update.rs already queries github.com/Hmbown/CodeWhale/releases/latest (fetch_latest_release / latest_release_tag, with proxy + China mirror fallback) and codewhale update --check-only works from the CLI.

Proposed scope for the in-app version (small):

1. A read-only /update (alias /check-updates) slash command in crates/tui/src/commands/ that calls the existing release-fetch logic, compares against the running version, and prints "up to date" / "vX.Y.Z available" plus the GitHub releases link (and the codewhale update hint).
2. To avoid the TUI depending on the CLI crate, lift the release-fetch helper into the shared codewhale-release crate (or duplicate the ~30-line GitHub fetch) so both call sites share it.
3. Optional: a one-time non-blocking check at startup that shows a subtle footer hint when a newer release exists (gated behind a config flag, off by default, network-respecting).

Leaving open as a ready-to-implement enhancement (not release-blocking for v0.8.51).

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