Worker thread + tray icon repaint fix

- processUtterance runs on a detached std.Thread; Qt event loop stays
  responsive during whisper/letta/melo (icon repaints, menu works)
- setState marshals to main thread via threading.async with thread-id check
- rapid double-click race guarded: state claimed before spawn
- icon: QPainter-drawn antialiased circle (dark rim, transparent bg) —
  bare pixmap.fill rendered blank on the KDE SNI host
- new qt libs: qbrush, qpainter, qrect, qt6zigthreading

👾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta Code <noreply@letta.com>

build.zig

@@ -16,11 +16,15 @@ pub fn build(b: *std.Build) void {
         "qcoreapplication",
         "qguiapplication",
         "qaction",
+        "qbrush",
         "qcolor",
         "qicon",
+        "qpainter",
         "qpixmap",
         "qmenu",
+        "qrect",
         "qsystemtrayicon",
+        "qt6zigthreading",
         "qwidget",
     };