fix: correct push protocol and WebSocket message handling

- Enable WebSocket compression (permessage-deflate) to match server
- Add `extension` field to push metadata (required by server)
- Fix push protocol: read metadata response before sending chunks,
  read per-chunk acks, handle "ok"/"next" server responses
- Drain pending push notifications before pushing in push command
- Handle server error field `err` in addition to `error`
- Add channel-based message dispatcher (readLoop) to safely multiplex
  push notifications, responses, and binary frames across goroutines

Previously pushes silently failed: the server rejected them but errors
were swallowed due to missing error field mapping and wrong message
ordering. The WebSocket connection also dropped every ~20s due to
unhandled compression frames.

Author: bpauli

internal/cmd/push.go

@@ -165,6 +165,17 @@ func (c *PushCmd) Run(ctx context.Context, flags *RootFlags) error {
 	}
 	defer sc.Close()
 
+	// Drain pending push notifications until "ready" before pushing.
+	for {
+		msg, err := sc.ReceivePush(ctx)
+		if err != nil {
+			return fmt.Errorf("receive push: %w", err)
+		}
+		if msg.Op == "ready" {
+			break
+		}
+	}
+
 	// Push changed/new files.
 	var pushCount int
 	for _, path := range pushPaths {

internal/cmd/push_test.go

@@ -25,6 +25,9 @@ func TestPushCmd_Success(t *testing.T) {
 
 	var pushedPaths []string
 	cleanup := mockSyncServer(t, vaults, func(conn *websocket.Conn) {
+		// Send ready so client drains initial sync.
+		conn.WriteJSON(map[string]any{"op": "ready", "version": int64(0)})
+
 		// Read push metadata.
 		var meta map[string]any
 		if err := conn.ReadJSON(&meta); err != nil {
@@ -37,6 +40,9 @@ func TestPushCmd_Success(t *testing.T) {
 		plainPath, _ := crypto.DecryptPath(key, encPath)
 		pushedPaths = append(pushedPaths, plainPath)
 
+		// Respond to metadata.
+		conn.WriteJSON(map[string]any{})
+
 		// Read binary chunk(s).
 		pieces := int(meta["pieces"].(float64))
 		for i := 0; i < pieces; i++ {
@@ -45,10 +51,9 @@ func TestPushCmd_Success(t *testing.T) {
 				t.Logf("read chunk: %v", err)
 				return
 			}
+			// Ack chunk.
+			conn.WriteJSON(map[string]any{"res": "ok"})
 		}
-
-		// Send ack.
-		conn.WriteJSON(map[string]any{"res": "ok"})
 	})
 	defer cleanup()
 
@@ -104,6 +109,9 @@ func TestPushCmd_DeletedFile(t *testing.T) {
 
 	var deletedPaths []string
 	cleanup := mockSyncServer(t, vaults, func(conn *websocket.Conn) {
+		// Send ready so client drains initial sync.
+		conn.WriteJSON(map[string]any{"op": "ready", "version": int64(10)})
+
 		// Read delete metadata.
 		var meta map[string]any
 		if err := conn.ReadJSON(&meta); err != nil {