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/agent/types.go

@@ -362,6 +362,9 @@ type WebRTCSession struct {
 	TaskID    string `json:"taskId,omitempty"`
 	FileName  string `json:"fileName,omitempty"`
 	FileSize  int64  `json:"fileSize,omitempty"`
+	// Quality target the daemon should aim for when transcoding. One of
+	// "2160p" | "1080p" | "720p" | "480p" | "original" | "" (defer to config).
+	Quality string `json:"quality,omitempty"`
 }
 
 // SyncResponse is returned by the server with all pending actions for the CLI.