desktop: wire packages/ui to in-process FFmpeg calls

[RichardBray]

Sub-issue of #133. Depends on packages/desktop shell issue.

Scope

• Replace CLI-trigger + Bun.serve HTTP hop (packages/ui/server.ts) with direct in-process calls from the desktop backend to @hance/core (probe, schema, presets, export-presets) and @hance/gpu/transcode functions.
• Introduce resolvable ffmpeg-path indirection: all spawn sites currently hardcode the "ffmpeg" string (packages/core/src/probe.ts, packages/ui/lib/transcode.ts, packages/gpu/src/export.ts). Needs to support pointing at a bundled binary later instead of system PATH.
• Desktop backend replicates the same "backend API" surface server.ts already exposes, just without HTTP.

Out of scope

• Bundling FFmpeg itself (#follow-up)