feat(desktop): open threads from external links

[mrmg]

What Changed

• Adds the t3code://app/#/<environmentId>/<threadId> deep-link format (and t3code-dev in development).
• Validates links strictly so only a scoped T3 thread route is accepted; unrelated custom-scheme traffic, including Clerk callback paths, is ignored.
• Routes launch arguments, second-instance activations, and macOS open-url events through the same handler.
• Reuses an existing desktop window when available, or retains the requested route until the backend is ready during cold startup.
• Documents the URI for external tooling.

Why

External tools can now target the exact T3 Code conversation a user needs—for example external device control, an issue notification, or an email link—without relying on browser window behavior. The URL carries only environment and thread IDs; the desktop app still requires that environment to already be configured and authenticated.

Verification

• vp test run --config apps/desktop/vite.config.ts … — 38 focused tests passed.
• vp run --filter @t3tools/desktop build — passed.
• tsgo --noEmit in apps/desktop — passed (two pre-existing suggestions only).

Checklist

• This PR is small and focused
• I explained what changed and why
• No visual UI changes require screenshots
• No animation or interaction video is applicable

Built with GPT-5.6-terra in T3 Code.

[mrmg]

Closing the fork-local duplicate. The upstream PR is pingdotgg#7383.