Improve crash diagnostics and isolate watcher-thread failures

Main-thread unhandled exceptions now show the traceback and log path; worker-thread exceptions log without touching Qt widgets. The download watcher catches its own loop failures and reports that it stopped instead of surfacing a blank error dialog. Adds a regression test for the watcher failure path.

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

src/boltclam/app.py

@@ -5,7 +5,9 @@ from __future__ import annotations
 import logging
 import os
 import sys
+import threading
 import time
+import traceback
 from pathlib import Path
 
 from PyQt6.QtCore import QEventLoop, QLockFile, QRectF, Qt, QTimer
@@ -18,7 +20,7 @@ from .auth import AuthService, CredentialStore, Session, SessionExpired
 from .clamav import ClamAVService
 from .config import AccountCache, SettingsStore
 from .context import AppContext
-from .logging_setup import configure as configure_logging
+from .logging_setup import configure as configure_logging, log_path
 from .quarantine import QuarantineService
 from .storage import Storage
 from .ui import brand, theme
@@ -73,10 +75,17 @@ def _install_exception_hook() -> None:
             sys.__excepthook__(exc_type, exc_value, exc_traceback)
             return
         logger.critical("Unhandled exception", exc_info=(exc_type, exc_value, exc_traceback))
+        # Qt widget calls are not safe from worker threads. Log there and
+        # leave the UI alone; the page's own status/toast reports the failure.
+        if threading.current_thread() is not threading.main_thread():
+            return
+        detail = "".join(traceback.format_exception(exc_type, exc_value, exc_traceback)).strip()
+        if len(detail) > 3200:
+            detail = "…\n" + detail[-3200:]
         try:
             QMessageBox.critical(
                 None, "BoltClam encountered an error",
-                f"{exc_type.__name__}: {exc_value}\n\nDetails were written to the BoltClam log.")
+                f"{detail}\n\nFull details: {log_path()}")
         except Exception:
             pass