Add WLED health monitoring, calibration test mode, and UI improvements

- Add background health checks (GET /json/info) with configurable interval per device
- Auto-detect LED count from WLED device on add (remove led_count from create API)
- Add calibration test mode: toggle edges on/off with colored LEDs via PUT endpoint
- Show WLED firmware version badge and LED count badge on device cards
- Add modal dirty tracking with discard confirmation on close/backdrop click
- Fix layout jump when modals open by compensating for scrollbar width
- Add state_check_interval to settings API and UI

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

server/src/wled_controller/static/locales/en.json

@@ -43,10 +43,11 @@
   "devices.wled_note2": "This controller sends pixel color data and controls brightness per device.",
   "device.name": "Device Name:",
   "device.name.placeholder": "Living Room TV",
-  "device.url": "WLED URL:",
+  "device.url": "URL:",
   "device.url.placeholder": "http://192.168.1.100",
   "device.led_count": "LED Count:",
   "device.led_count.hint": "Number of LEDs configured in your WLED device",
+  "device.led_count.hint.auto": "Auto-detected from WLED device",
   "device.button.add": "Add Device",
   "device.button.start": "Start",
   "device.button.stop": "Stop",
@@ -69,22 +70,23 @@
   "device.metrics.target_fps": "Target FPS",
   "device.metrics.frames": "Frames",
   "device.metrics.errors": "Errors",
+  "device.health.online": "WLED Online",
+  "device.health.offline": "WLED Offline",
+  "device.health.checking": "Checking...",
   "settings.title": "Device Settings",
   "settings.brightness": "Brightness:",
   "settings.brightness.hint": "Global brightness for this WLED device (0-100%)",
   "settings.url.hint": "IP address or hostname of your WLED device",
   "settings.button.cancel": "Cancel",
+  "settings.health_interval": "Health Check Interval (s):",
+  "settings.health_interval.hint": "How often to check the WLED device status (5-600 seconds)",
   "settings.button.save": "Save Changes",
   "settings.saved": "Settings saved successfully",
   "settings.failed": "Failed to save settings",
   "calibration.title": "LED Calibration",
-  "calibration.description": "Configure how your LED strip is mapped to screen edges. Use test buttons to verify each edge lights up correctly.",
+  "calibration.description": "Configure how your LED strip is mapped to screen edges. Click an edge to toggle test mode.",
   "calibration.preview.screen": "Screen",
-  "calibration.preview.top": "Top:",
-  "calibration.preview.right": "Right:",
-  "calibration.preview.bottom": "Bottom:",
-  "calibration.preview.left": "Left:",
-  "calibration.preview.leds": "LEDs",
+  "calibration.preview.click_hint": "Click an edge to toggle test LEDs on/off",
   "calibration.start_position": "Starting Position:",
   "calibration.position.bottom_left": "Bottom Left",
   "calibration.position.bottom_right": "Bottom Right",
@@ -100,21 +102,16 @@
   "calibration.leds.bottom": "Bottom LEDs:",
   "calibration.leds.left": "Left LEDs:",
   "calibration.total": "Total LEDs:",
-  "calibration.test": "Test Edges (lights up each edge):",
-  "calibration.test.top": "Top",
-  "calibration.test.right": "Right",
-  "calibration.test.bottom": "Bottom",
-  "calibration.test.left": "Left",
   "calibration.button.cancel": "Cancel",
-  "calibration.button.save": "Save Calibration",
+  "calibration.button.save": "Save",
   "calibration.saved": "Calibration saved successfully",
   "calibration.failed": "Failed to save calibration",
-  "calibration.testing": "Testing {edge} edge...",
   "server.healthy": "Server online",
   "server.offline": "Server offline",
   "error.unauthorized": "Unauthorized - please login",
   "error.network": "Network error",
   "error.unknown": "An error occurred",
+  "modal.discard_changes": "You have unsaved changes. Discard them?",
   "confirm.title": "Confirm Action",
   "confirm.yes": "Yes",
   "confirm.no": "No"