fix(preview): adaptive streaming to fix blur, bandwidth, and latency

[ArgaFairuz]

Rework the browser preview pipeline into a Chrome-Remote-Desktop-style adaptive stream:

• Capture at native viewport resolution; display fit-scale is CSS-only. Client renders frames 1:1 (canvas backing store follows frame size), removing the upscaling that made the preview permanently blurry.
• Prefer VP9 in quantizer mode, negotiated with the client's decode support at stream start (VP8 bitrate-mode fallback): cheap per-frame quality during motion, raised further under congestion.
• Static top-off: when the screencast goes quiet for 300ms, push one lossless PNG screenshot through the encoder near-losslessly so text sharpens right after motion stops; idle pages cost ~0 bandwidth. Deferred while the channel is congested and abandoned if motion resumes, so large refreshes never add latency.
• Reliable ordered DataChannel (VP8/VP9 delta chains cannot tolerate loss/reorder); latency is bounded by source-side frame dropping with a small 64KB backpressure window instead — queued bytes are display lag, so the stream degrades to coarser-but-current, never stale.
• Fragment frames over 64KB (packet type 2) to stay under SCTP message-size limits; packet header now carries a codec byte.
• Keyframes on demand only: channel open, encoder reconfigure, or the new preview:browser-stream-keyframe request (PLI equivalent) that also works on still pages; no periodic keyframe bursts.
• Rate-limit encoding to the configured framerate at the CDP feeder with a trailing edge — the last frame of a damage burst is always encoded, keeping input feedback immediate while capping CPU and bandwidth (screencast fires at compositor rate, up to 60fps).
• Add a 5s grace period for transient ICE 'disconnected' states on flaky networks (mobile/tunnel) — fixes the endless loading/reconnect loop when the preview is accessed remotely.
• Drop audio chunks during congestion; the client's playback scheduler handles gaps cleanly.