Streaming overlay: chat GUI for agent responses

- overlay.zig: frameless translucent panel (fixed bg + rounded corners,
  only bubbles scroll), drag handle, hover-to-full-opacity, top-right spawn
- chat layout: per-message QLabel bubbles (user purple / agent green /
  tool steps grey mono), transcript retained across turns, End-conversation
  resets
- agent reply streams into its own growing bubble (re-render per chunk)
- basic markdown in all messages: **bold**, *italic*, `code`, HTML-escaped
- tool steps truncated to 90 chars, click to expand/collapse
- polite autoscroll: pins to bottom unless user scrolled up
- letta.zig: streamInfer() — --output-format stream-json, per-event callback
  (tool calls, returns, reply chunks), full reply still returned for TTS
- whisper: --language auto (no more hardcoded English)
- fix: stderr_log leak crashing Quit via debug allocator

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

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

build.zig

@@ -16,8 +16,19 @@ pub fn build(b: *std.Build) void {
         "qcoreapplication",
         "qguiapplication",
         "qaction",
+        "qabstractscrollarea",
+        "qabstractslider",
+        "qboxlayout",
+        "qcursor",
+        "qevent",
+        "qlabel",
+        "qlayout",
+        "qobject",
         "qbrush",
         "qcolor",
+        "qpoint",
+        "qscreen",
+        "qscrollarea",
         "qicon",
         "qpainter",
         "qpixmap",
@@ -25,6 +36,7 @@ pub fn build(b: *std.Build) void {
         "qrect",
         "qsystemtrayicon",
         "qt6zigthreading",
+        "qtextedit",
         "qwidget",
     };