fix: remove unused imports and bare exception aliases (pyflakes F401/F841)

Remove unused imports across 86 files in src/, web_interface/, test/,
and scripts/ using autoflake. No logic changes — only dead import
statements and unused names in from-imports are removed.

Also remove bare exception aliases where the variable is never
referenced in the handler body:
- src/cache/disk_cache.py: except (IOError, OSError, PermissionError) as e
- src/cache_manager.py: except (OSError, IOError, PermissionError) as perm_error
- src/plugin_system/resource_monitor.py: except Exception as e
- web_interface/app.py: except Exception as read_err

86 files changed, 205 lines removed, 18 pre-existing test failures unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/test_text_helper.py

@@ -5,7 +5,7 @@ Tests text rendering, font loading, and text positioning utilities.
 """
 
 import pytest
-from unittest.mock import MagicMock, patch, Mock
+from unittest.mock import MagicMock, patch
 from PIL import Image, ImageDraw, ImageFont
 from src.common.text_helper import TextHelper