Add a portable MCP App interface

[tulayha]

Add a compact MCP App for compatible agent hosts to present common VidXP workflows directly inside a conversation.

The first useful version should let users select or upload videos, follow indexing progress, inspect evidence boards and keyframes, and open or download generated clips.

It should reuse VidXP's existing MCP tools, resources, and media lifecycle rather than creating a separate application backend. The full browser library can remain the primary surface for broader media management.

[Haseeb-1698]

I'd like to pick up the rest of this one if it's still open.

The evidence-review widget from #109 already covers part of the "first useful version" list: uploading (create_media_upload) and inspecting evidence boards, keyframes and clips (present_job_evidence) are both wired to ui://vidxp/evidence-review-v1.html. What's still missing is selecting an existing video from the library, and following indexing progress.

Both look reachable without a separate backend, which is what the issue asks for. list_media, get_media, list_jobs, get_job and wait_job already exist as tools — so it's a matter of extending src/vidxp/assets/mcp_app/index.html and attaching _mcp_app_tool_meta to the read-only ones, reusing the existing media and job lifecycle rather than adding surface.

Happy to scope it differently if you have a direction in mind. It also splits cleanly if you'd prefer smaller PRs: library selection first, indexing progress second.