feat(stream): per-session quality cap from web

Adds WebRTCSession.Quality to the sync payload so the daemon can pick a
MaxHeight + bitrate per session instead of using the global config cap.

resolveQualityCap() maps the label to a (height, b:v) pair and
buildStreamSource() promotes a passthrough decision to ActionTranscodeVideo
when the source resolution exceeds the cap (4K source on a phone client
with quality="720p" must transcode, not pass-through).

Also lands the transcode-on-by-default fix for legacy configs without a
[downloads.transcode] section so existing installs pick up h264+aac
fallback for HEVC/AC3 content without re-running setup.

internal/cmd/daemon.go

@@ -457,6 +457,7 @@ func runDaemonStart() error {
 				FilePath:   filePath,
 				FileName:   sess.FileName,
 				FileSize:   sess.FileSize,
+				Quality:    sess.Quality,
 				ICEServers: engine.BuildICEServers(cfg.Download.WebRTC),
 				Signal:     agentClient,
 				Logger:     stdLogger{},