Add internationalization (i18n) support with English and Russian translations

Implemented localization system similar to the media-server project pattern:
- Created locale JSON files for English (en.json) and Russian (ru.json)
- Added complete translations for all UI elements, buttons, labels, and messages
- Implemented locale management system with browser locale detection
- Added language selector dropdown in header
- Applied data-i18n, data-i18n-title, and data-i18n-placeholder attributes
- Translations stored in localStorage and persist across sessions
- Automatic language detection from browser settings
- All dynamic content (displays, devices, modals) now uses translation function

Translations cover:
- Authentication (login/logout)
- Displays (layout visualization, cards, labels)
- Devices (management, status, actions)
- Settings modal (brightness, device configuration)
- Calibration modal (LED mapping, testing)
- Error messages and notifications
- Server status and version information

The implementation uses:
- Simple t(key, params) translation function with parameter substitution
- Async locale loading from /static/locales/{locale}.json
- updateAllText() to refresh all UI elements when language changes
- Fallback to English if translation file fails to load

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

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

@@ -0,0 +1,111 @@
+{
+  "app.title": "WLED Screen Controller",
+  "app.version": "Version:",
+  "theme.toggle": "Toggle theme",
+  "locale.change": "Change language",
+  "auth.login": "Login",
+  "auth.logout": "Logout",
+  "auth.authenticated": "Authenticated",
+  "auth.title": "Login to WLED Controller",
+  "auth.message": "Please enter your API key to authenticate and access the WLED Screen Controller.",
+  "auth.label": "API Key:",
+  "auth.placeholder": "Enter your API key...",
+  "auth.hint": "Your API key will be stored securely in your browser's local storage.",
+  "auth.button.cancel": "Cancel",
+  "auth.button.login": "Login",
+  "auth.error.required": "Please enter an API key",
+  "auth.success": "Logged in successfully!",
+  "auth.logout.confirm": "Are you sure you want to logout?",
+  "auth.logout.success": "Logged out successfully",
+  "auth.please_login": "Please login to view",
+  "displays.title": "Available Displays",
+  "displays.layout": "Display Layout",
+  "displays.information": "Display Information",
+  "displays.legend.primary": "Primary Display",
+  "displays.legend.secondary": "Secondary Display",
+  "displays.badge.primary": "Primary",
+  "displays.badge.secondary": "Secondary",
+  "displays.resolution": "Resolution:",
+  "displays.position": "Position:",
+  "displays.index": "Display Index:",
+  "displays.loading": "Loading displays...",
+  "displays.none": "No displays available",
+  "displays.failed": "Failed to load displays",
+  "devices.title": "WLED Devices",
+  "devices.add": "Add New Device",
+  "devices.loading": "Loading devices...",
+  "devices.none": "No devices configured",
+  "devices.failed": "Failed to load devices",
+  "devices.wled_config": "WLED Configuration:",
+  "devices.wled_note": "Configure your WLED device (effects, segments, color order, power limits, etc.) using the",
+  "devices.wled_link": "official WLED app",
+  "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.placeholder": "http://192.168.1.100",
+  "device.led_count": "LED Count:",
+  "device.led_count.hint": "Number of LEDs configured in your WLED device",
+  "device.button.add": "Add Device",
+  "device.button.start": "Start",
+  "device.button.stop": "Stop",
+  "device.button.settings": "Settings",
+  "device.button.calibrate": "Calibrate",
+  "device.button.remove": "Remove",
+  "device.status.connected": "Connected",
+  "device.status.disconnected": "Disconnected",
+  "device.status.error": "Error",
+  "device.status.processing": "Processing",
+  "device.status.idle": "Idle",
+  "device.fps": "FPS:",
+  "device.display": "Display:",
+  "device.remove.confirm": "Are you sure you want to remove this device?",
+  "device.added": "Device added successfully",
+  "device.removed": "Device removed successfully",
+  "device.started": "Processing started",
+  "device.stopped": "Processing stopped",
+  "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.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.preview.screen": "Screen",
+  "calibration.preview.top": "Top:",
+  "calibration.preview.right": "Right:",
+  "calibration.preview.bottom": "Bottom:",
+  "calibration.preview.left": "Left:",
+  "calibration.preview.leds": "LEDs",
+  "calibration.start_position": "Starting Position:",
+  "calibration.position.bottom_left": "Bottom Left",
+  "calibration.position.bottom_right": "Bottom Right",
+  "calibration.position.top_left": "Top Left",
+  "calibration.position.top_right": "Top Right",
+  "calibration.direction": "Direction:",
+  "calibration.direction.clockwise": "Clockwise",
+  "calibration.direction.counterclockwise": "Counterclockwise",
+  "calibration.leds.top": "Top LEDs:",
+  "calibration.leds.right": "Right LEDs:",
+  "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.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"
+}