fix: harden restart_claude_desktop across platforms (#33)

- Linux (and any non-desktop platform): refuse instead of 'pkill claude',
  which substring-matched running Claude Code CLI sessions and relaunched
  the CLI, not a desktop app. New restart_supported() gates the button.
- macOS: wait (<=5s) for the old instance to exit before 'open -a Claude'
  so the relaunch can't re-activate the dying process. Runs off the UI
  thread via a RestartWorker.
- Windows: verify the Start-menu shortcut exists BEFORE taskkill, so an
  MSIX/Store install is never killed without a relaunch path.

Closes #33

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bcc_core.py

@@ -1837,8 +1837,41 @@ def _run_quiet(cmd: list[str]) -> None:
         subprocess.run(cmd, capture_output=True)
 
 
+def restart_supported() -> bool:
+    """
+    True only where restarting Claude Desktop makes sense (macOS, Windows).
+    There is no official Claude Desktop for Linux, and the obvious binary
+    name there ("claude") is the Claude Code CLI — killing or spawning it
+    would be actively harmful. GUI callers gate the Restart button on this.
+    """
+    return sys.platform == "darwin" or sys.platform.startswith("win")
+
+
+_MACOS_QUIT_WAIT_S = 5.0
+
+
+def _macos_claude_running() -> bool:
+    try:
+        return subprocess.run(["pgrep", "-x", "Claude"], capture_output=True).returncode == 0
+    except OSError:
+        return False
+
+
 def _restart_claude_desktop_macos() -> RestartResult:
     _run_quiet(["pkill", "-x", "Claude"])
+    # Wait for the old instance to actually exit: `open -a` against a dying
+    # process can merely re-activate it, and the config is only re-read on a
+    # true relaunch. Blocks up to _MACOS_QUIT_WAIT_S — callers run this off
+    # the UI thread (see RestartWorker in bcc.py).
+    deadline = time.monotonic() + _MACOS_QUIT_WAIT_S
+    while _macos_claude_running():
+        if time.monotonic() > deadline:
+            return RestartResult(
+                False,
+                f"Claude Desktop didn't quit within {_MACOS_QUIT_WAIT_S:.0f}s — "
+                "quit it manually, then reopen it.",
+            )
+        time.sleep(0.15)
     try:
         result = subprocess.run(["open", "-a", "Claude"], capture_output=True, text=True)
     except OSError as e:
@@ -1855,8 +1888,18 @@ def _claude_windows_start_menu_shortcut() -> Path:
 
 
 def _restart_claude_desktop_windows() -> RestartResult:
-    _run_quiet(["taskkill", "/IM", "Claude.exe", "/F"])
     shortcut = _claude_windows_start_menu_shortcut()
+    if not shortcut.is_file():
+        # Checked BEFORE killing: an MSIX/Store install has no Start-menu .lnk
+        # at this path, and killing without a relaunch path would leave the
+        # user with no running Claude at all.
+        return RestartResult(
+            False,
+            f"Claude's Start-menu shortcut wasn't found ({shortcut}). "
+            "If Claude Desktop is installed from the Microsoft Store, "
+            "quit and reopen it manually.",
+        )
+    _run_quiet(["taskkill", "/IM", "Claude.exe", "/F"])
     try:
         # `cmd /c start "" <target>` launches detached, the same as double-clicking
         # the Start-menu shortcut, and returns immediately.
@@ -1873,32 +1916,19 @@ def _restart_claude_desktop_windows() -> RestartResult:
     return RestartResult(True, "Claude Desktop restarted.")
 
 
-def _restart_claude_desktop_linux() -> RestartResult:
-    _run_quiet(["pkill", "claude"])
-    try:
-        # The Linux launcher is the app itself (no "open"-style helper), so it
-        # has to be started detached rather than waited on.
-        subprocess.Popen(
-            ["claude"],
-            stdin=subprocess.DEVNULL,
-            stdout=subprocess.DEVNULL,
-            stderr=subprocess.DEVNULL,
-            start_new_session=True,
-        )
-    except OSError as e:
-        return RestartResult(False, f"Couldn't launch Claude Desktop: {e}")
-    return RestartResult(True, "Claude Desktop restarted.")
-
-
 def restart_claude_desktop() -> RestartResult:
     """
     Kill and relaunch the Claude Desktop app so a freshly saved config takes
     effect. The app not currently running is NOT a failure -- pkill/taskkill
     exiting non-zero just means "nothing to kill", and we go straight to
     relaunching. Only a failed relaunch is reported as success=False.
+
+    macOS blocks for up to _MACOS_QUIT_WAIT_S while the old instance exits —
+    run off the UI thread. Unsupported platforms (see restart_supported())
+    refuse without touching any process.
     """
     if sys.platform == "darwin":
         return _restart_claude_desktop_macos()
     if sys.platform.startswith("win"):
         return _restart_claude_desktop_windows()
-    return _restart_claude_desktop_linux()
+    return RestartResult(False, "Restarting Claude Desktop isn't supported on this platform.")