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/index.html

@@ -21,6 +21,18 @@
             </div>
             <div id="hud" hidden></div>
             <div id="toast" hidden></div>
+            <div id="export-warn" hidden>
+                <div class="ew-backdrop"></div>
+                <div class="ew-box" role="dialog" aria-modal="true" aria-labelledby="ew-title">
+                    <div id="ew-title" class="ew-title">Software encoding — export will be slow</div>
+                    <div id="ew-body" class="ew-body"></div>
+                    <div class="ew-actions">
+                        <button id="ew-cancel">Cancel</button>
+                        <button id="ew-continue" class="primary">Continue anyway</button>
+                    </div>
+                    <label class="ew-opt"><input type="checkbox" id="ew-dontask"> Don't warn again this session</label>
+                </div>
+            </div>
         </div>
 
         <div id="transport">