feat: add timezone support for schedules and dim schedule feature (#218)

* feat: add timezone support for schedules and dim schedule feature

- Fix timezone handling in _check_schedule() to use configured timezone
  instead of system time (addresses schedule offset issues)
- Add dim schedule feature for automatic brightness dimming:
  - New dim_schedule config section with brightness level and time windows
  - Smart interaction: dim schedule won't turn display on if it's off
  - Supports both global and per-day modes like on/off schedule
- Add set_brightness() and get_brightness() methods to DisplayManager
  for runtime brightness control
- Add REST API endpoints: GET/POST /api/v3/config/dim-schedule
- Add web UI for dim schedule configuration in schedule settings page

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix: normalize per-day mode and validate dim_brightness input

- Normalize mode string in _check_dim_schedule to handle both "per-day"
  and "per_day" variants
- Add try/except around dim_brightness int conversion to handle invalid
  input gracefully

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: improve error handling in brightness and dim schedule endpoints

- display_manager.py: Add fail-fast input validation, catch specific
  exceptions (AttributeError, TypeError, ValueError), add [BRIGHTNESS]
  context tags, include stack traces in error logs
- api_v3.py: Catch specific config exceptions (FileNotFoundError,
  JSONDecodeError, IOError), add [DIM SCHEDULE] context tags for
  Pi debugging, include stack traces

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

config/config.template.json

@@ -43,6 +43,50 @@
             }
         }
     },
+    "dim_schedule": {
+        "enabled": false,
+        "dim_brightness": 30,
+        "mode": "global",
+        "start_time": "20:00",
+        "end_time": "07:00",
+        "days": {
+            "monday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            },
+            "tuesday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            },
+            "wednesday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            },
+            "thursday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            },
+            "friday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            },
+            "saturday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            },
+            "sunday": {
+                "enabled": true,
+                "start_time": "20:00",
+                "end_time": "07:00"
+            }
+        }
+    },
     "timezone": "America/Chicago",
     "location": {
         "city": "Dallas",