fix(hooks): handle missing tkinter cleanly in flashbang.sh

tkinter was imported at module level, before the try/except that was
meant to catch a missing display or toolkit. On a Python install without
the (often separately packaged) tkinter module, the script crashed
instead of falling back to notify-send. The import is now wrapped in its
own try/except with the same fallback.

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

shared/hooks/flashbang.sh

@@ -16,7 +16,6 @@ import re
 import shutil
 import subprocess
 import sys
-import tkinter as tk
 
 
 def notify_fallback() -> None:
@@ -25,6 +24,13 @@ def notify_fallback() -> None:
         subprocess.run([notify_send, "AI assistant requires attention"], check=False)
 
 
+try:
+    import tkinter as tk
+except ImportError:
+    notify_fallback()
+    sys.exit(0)
+
+
 def monitor_bounds(primary_only: bool) -> tuple[int, int, int, int]:
     root = tk.Tk()
     root.withdraw()