feat(desktop): add manual "Check for Updates" button in web settings

[AriajSarkar]

What Changed

• Add checkForUpdate IPC channel (DesktopUpdateCheckResult type, DesktopBridge.checkForUpdate())
• Wire UPDATE_CHECK_CHANNEL in preload + main process handler (reuses existing checkForUpdates()
• Add "Check for Updates" button in Settings → About with contextual labels, error handling, and last-checked timestamp
• Add canCheckForUpdate / getCheckForUpdateButtonLabel pure logic helpers
• Guard for unconfigured updater in dev/local builds
• 15 new unit tests

Why

The desktop app checks for updates automatically and has a native menu "Check for Updates…", but there's no way to trigger a check from the web UI. Users may not discover the menu item. This adds a simple, focused entry point in Settings → About.

Closes #1107 (already closed)

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Local Test:
v0.0.10 -> v0.0.11

PS C:\all\Open-Source\t3code> $env:T3CODE_DESKTOP_UPDATE_REPOSITORY="pingdotgg/t3code"
>> bun run dist:desktop:win

Note

Add manual "Check for Updates" button to desktop settings page

• Adds a DesktopUpdateCheckSection component in _chat.settings.tsx that renders an 'Updates' card in the About section of desktop settings, with a button whose label and enabled state reflect the current update status.
• Exposes checkForUpdate() on window.desktopBridge via a new desktop:update-check IPC channel, returning {checked, state} so the UI can refresh the cached update state after each check.
• Adds canCheckForUpdate and getCheckForUpdateButtonLabel helpers in desktopUpdate.logic.ts to derive button state from DesktopUpdateState; the button also handles download and install actions depending on update state.
• Update state is fetched and cached via React Query (desktopUpdateReactQuery.ts) and kept live via onUpdateState bridge events.

^{Macroscope summarized e840966.}

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