fix(project): Non-blocking project switch with stale-ui tint

When switching projects, the previous implementation ran the entire
save/load/refresh sequence on the main thread. With large project files
or slow disks, this caused the UI to freeze for several seconds.

Fix:
- _switch_project now returns immediately after setting flags; the
  actual work runs in a daemon thread (_do_project_switch)
- New is_project_stale() property returns True while a switch is queued
  or running; the GUI renders an amber/yellow tint overlay to signal
  the controller state lags the user's last click
- AI ops are gated: _api_generate returns HTTP 409, _handle_generate_send
  and _handle_md_only early-return with ai_status feedback, all when
  is_project_stale() is true
- Queued switches (clicking project A then B in rapid succession) are
  coalesced: B replaces A as the target; once A completes, B is
  triggered automatically via the finally branch in _do_project_switch
- New state fields: _project_switch_in_progress, _project_switch_pending_path,
  _project_switch_thread, _project_switch_lock
- AppController state class attributes use hasattr guard for _app to
  keep the controller usable standalone in tests/headless mode

UX:
- Render loop keeps drawing during the switch
- User can still scroll, switch tabs, browse files
- Amber tint + popup explains what's happening and that AI ops are paused
- ai_status shows the target project name

Tests:
- _wait_for_switch helper added for the new async switch flow
- All 7 existing switch tests updated to call _wait_for_switch
- 2 new tests:
  - test_switch_project_non_blocking: verifies _switch_project returns
    in <0.2s and is_project_stale() is True during the switch
  - test_api_generate_blocked_while_stale: verifies _api_generate
    raises HTTPException(409) while a switch is in progress

All 33 related tests pass.

tests/test_project_switch_persona_preset.py

@@ -1,5 +1,6 @@
 import os
 import json
+import time
 import pytest
 import tempfile
 import shutil
@@ -11,6 +12,19 @@ from src import presets, tool_presets
 from src import project_manager
 
 
+def _wait_for_switch(ctrl, timeout: float = 2.0) -> None:
+    """Polls until any background project switch thread completes."""
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        with ctrl._project_switch_lock:
+            in_progress = ctrl._project_switch_in_progress
+        thread = ctrl._project_switch_thread
+        if not in_progress and (thread is None or not thread.is_alive()):
+            return
+        time.sleep(0.02)
+    raise AssertionError("Project switch did not complete within timeout")
+
+
 def _setup_two_projects(tmp_path):
     project_a_dir = tmp_path / "project_a"
     project_a_dir.mkdir()
@@ -57,6 +71,7 @@ def test_switch_project_resets_invalid_persona(tmp_path, monkeypatch):
     ctrl.ui_active_persona = "PersonaA"
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert "PersonaA" not in ctrl.personas
     assert "PersonaB" in ctrl.personas
@@ -80,6 +95,7 @@ def test_switch_project_resets_invalid_preset(tmp_path, monkeypatch):
     assert ctrl.ui_project_preset_name == "PresetA"
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert "PresetA" not in ctrl.presets
     assert "PresetB" in ctrl.presets
@@ -111,6 +127,7 @@ def test_switch_project_resets_invalid_tool_preset(tmp_path, monkeypatch):
     assert "ToolA" in ctrl.tool_presets
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert "ToolA" not in ctrl.tool_presets
     assert "ToolB" in ctrl.tool_presets
@@ -135,6 +152,7 @@ def test_switch_project_preserves_global_preset(tmp_path, monkeypatch):
     original_global = ctrl.ui_global_preset_name
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert ctrl.ui_global_preset_name == original_global
 
@@ -213,8 +231,65 @@ paths = [
     assert any("gte_hello" in f.path for f in ctrl.context_files)
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert len(ctrl.context_files) == 2
     assert all("gencpp" in f.path for f in ctrl.context_files)
     assert not any("forth" in f.path for f in ctrl.context_files)
     assert not any("gte_hello" in f.path for f in ctrl.context_files)
+
+
+def test_switch_project_non_blocking(tmp_path, monkeypatch):
+    """The switch returns immediately; the heavy work runs in a background
+    thread. UI calls see is_project_stale() == True while it's running."""
+    project_a_path, project_b_path = _setup_two_projects(tmp_path)
+
+    ctrl = AppController()
+    monkeypatch.setattr(ctrl, "_rebuild_rag_index", lambda: None)
+    monkeypatch.setattr(ctrl, "_flush_to_project", lambda: None)
+
+    ctrl.active_project_path = str(project_a_path)
+    ctrl.project = project_manager.load_project(str(project_a_path))
+    ctrl.preset_manager = presets.PresetManager(Path(project_a_path).parent)
+    ctrl.tool_preset_manager = tool_presets.ToolPresetManager(Path(project_a_path).parent)
+    ctrl.persona_manager = PersonaManager(Path(project_a_path).parent)
+    ctrl._refresh_from_project()
+
+    started = time.time()
+    ctrl._switch_project(str(project_b_path))
+    elapsed = time.time() - started
+
+    assert elapsed < 0.2, f"_switch_project blocked the caller for {elapsed:.2f}s"
+    assert ctrl.is_project_stale()
+    _wait_for_switch(ctrl)
+    assert not ctrl.is_project_stale()
+    assert ctrl.active_project_path == str(project_b_path)
+
+
+def test_api_generate_blocked_while_stale(tmp_path, monkeypatch):
+    """_api_generate returns 409 when the project switch is in progress."""
+    project_a_path, project_b_path = _setup_two_projects(tmp_path)
+
+    ctrl = AppController()
+    monkeypatch.setattr(ctrl, "_rebuild_rag_index", lambda: None)
+    monkeypatch.setattr(ctrl, "_flush_to_project", lambda: None)
+
+    ctrl.active_project_path = str(project_a_path)
+    ctrl.project = project_manager.load_project(str(project_a_path))
+    ctrl.preset_manager = presets.PresetManager(Path(project_a_path).parent)
+    ctrl.tool_preset_manager = tool_presets.ToolPresetManager(Path(project_a_path).parent)
+    ctrl.persona_manager = PersonaManager(Path(project_a_path).parent)
+    ctrl._refresh_from_project()
+
+    ctrl._switch_project(str(project_b_path))
+    assert ctrl.is_project_stale()
+    from fastapi import HTTPException
+    from src.app_controller import _api_generate
+    from src.models import GenerateRequest
+    req = GenerateRequest(prompt="hello")
+    with pytest.raises(HTTPException) as exc_info:
+        _api_generate(ctrl, req)
+    assert exc_info.value.status_code == 409
+
+    _wait_for_switch(ctrl)
+    assert not ctrl.is_project_stale()