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_error_handling.py

@@ -1,16 +1,10 @@
-import pytest
 import logging
 import json
-import tempfile
-from pathlib import Path
-from src.exceptions import CacheError, ConfigError, PluginError, DisplayError, LEDMatrixError
+from src.exceptions import CacheError, ConfigError, PluginError, DisplayError
 from src.common.error_handler import (
     handle_file_operation,
     handle_json_operation,
-    safe_execute,
-    retry_on_failure,
-    log_and_continue,
-    log_and_raise
+    safe_execute
 )
 
 class TestCustomExceptions: