perf(call): H.264 + contentHint + 30fps default for screen-share

The real cost in the "frame by frame" stutter wasn't the custom capture
path — it was LiveKit re-encoding via VP9 software with L3T3_KEY SVC
(three spatial × three temporal layers, all CPU). Switching the
per-publish codec to H.264 lets Chromium's hardware encoder take over
on Windows and sidesteps the SVC mode entirely (H.264 has no SVC).
Also pushes `contentHint = 'detail'` on the track — setScreenShareEnabled
does this internally, the manual publishTrack paths had been missing it,
which changes how the encoder allocates its frame budget for static UI
content.

Auto preset default framerate 60 → 30. 60fps desktop share burns three
full-res encodes per frame at sizes up to 4K; 30 is what getDisplayMedia
practically delivers anyway.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src/lib/screenShareSettings.ts

@@ -46,7 +46,7 @@ export interface PresetParams {
 }
 
 const PRESET_PARAMS: Record<ScreenSharePreset, PresetParams> = {
-  auto: { dims: null, framerate: 60, bitrateKbps: 8000, label: 'Auto (Original)' },
+  auto: { dims: null, framerate: 30, bitrateKbps: 8000, label: 'Auto (Original)' },
   '720p30': {
     dims: { width: 1280, height: 720 },
     framerate: 30,