feat(stream)!: retire WebRTC, HLS-only, bump 0.9.4

Drops the custom WebRTC DataChannel pipeline + pion deps + WSS signaling
client + wire framing. Every in-browser playback now uses HLS over HTTP
from the daemon (Tailscale/LAN/UPnP). Browser P2P never re-enabled.

Wire renames (incompatible with web < 2026-05-26): agent.WebRTCSession
=> agent.StreamSession, SyncResponse.WebRTCSessions (JSON: webrtcSessions)
=> StreamSessions (JSON: streamSessions). MIN_AGENT_VERSION is bumped
to 0.9.4 on the web side so older agents see an upgrade card.

Also fixes the libx264 'VBV bitrate > level limit' abort by clamping
the encoder bitrate to the effective output height instead of the
requested label (carried over from the prior 0.9.3 unreleased work).

The seed_file vertical (mode=seed_file handler + engine.SeedFile) was
retired with the in-browser P2P player. [downloads.webrtc] config block
deleted; existing TOML files with the section still parse fine.

internal/agent/sync.go

@@ -29,7 +29,7 @@ type SyncClient struct {
 	OnNewTasks       func(tasks []Task)
 	OnControl        func(action, taskID string, deleteFiles bool)
 	OnStreamRequest  func(req StreamRequest)
-	OnWebRTCSession  func(sess WebRTCSession)
+	OnStreamSession  func(sess StreamSession)
 	OnUpgrade        func(version string)
 	OnScan           func()
 	OnWatchingChange func(watching bool)
@@ -199,10 +199,10 @@ func (sc *SyncClient) processResponse(resp *SyncResponse) {
 		}
 	}
 
-	// WebRTC streaming sessions
-	for _, ws := range resp.WebRTCSessions {
-		if sc.OnWebRTCSession != nil {
-			sc.OnWebRTCSession(ws)
+	// HLS streaming sessions.
+	for _, ws := range resp.StreamSessions {
+		if sc.OnStreamSession != nil {
+			sc.OnStreamSession(ws)
 		}
 	}