Export: fall back to VP9/AV1 when H.264 unavailable and warn about software encoding

Firefox on Linux exposes VideoEncoder but reports every avc1.* config as
unsupported (OpenH264 is decode-only), so pickVideoConfig returned null and
export threw "no supported H.264 configuration found". Probe VP9/AV1 families
after H.264 and pick the first profile that isConfigSupported and does not
reorder (B-frame) — same 12-frame ordering test. Map the chosen family to
mp4-muxer's video codec (avc/vp9/av1).

Before any rendering, probe the codec for the chosen preset and show a
modal when we fell off H.264 onto a software VP9/AV1 path — that's what the
user felt as "super slow". The modal names the codec, explains render vs
encode cost, suggests Chromium/Chrome for hardware H.264 (or 720p/lower
preset), and offers Continue anyway / Cancel (Esc/backdrop also cancel) plus
"Don't warn again this session". Injected pick is threaded through
Exporter.export({ videoPick }) and exportSegment so we don't probe twice.

Co-Authored-By: Claude <noreply@anthropic.com>

flow-state/package-lock.json

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