Add adaptive FPS and honest device reachability during streaming

DDP uses fire-and-forget UDP, so when a WiFi device becomes overwhelmed
by sustained traffic, sends appear successful while the device is
actually unreachable. This adds:

- HTTP liveness probe (GET /json/info, 2s timeout) every 10s during
  streaming, exposed as device_streaming_reachable in target state
- Adaptive FPS (opt-in): exponential backoff when device is unreachable,
  gradual recovery when it stabilizes — finds sustainable send rate
- Honest health checks: removed the lie that forced device_online=true
  during streaming; now runs actual health checks regardless
- Target editor toggle, FPS display shows effective rate when throttled,
  health dot reflects streaming reachability, red highlight when
  unreachable
- Auto-backup scheduling support in settings modal

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

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

@@ -169,6 +169,7 @@
   "device.metrics.device_fps": "Device refresh rate",
   "device.health.online": "Online",
   "device.health.offline": "Offline",
+  "device.health.streaming_unreachable": "Unreachable during streaming",
   "device.health.checking": "Checking...",
   "device.tutorial.start": "Start tutorial",
   "device.tip.metadata": "Device info (LED count, type, color channels) is auto-detected from the device",
@@ -912,6 +913,8 @@
   "targets.brightness_vs.none": "None (device brightness)",
   "targets.min_brightness_threshold": "Min Brightness Threshold:",
   "targets.min_brightness_threshold.hint": "Effective output brightness (pixel brightness × device/source brightness) below this value turns LEDs off completely (0 = disabled)",
+  "targets.adaptive_fps": "Adaptive FPS:",
+  "targets.adaptive_fps.hint": "Automatically reduce send rate when the device becomes unresponsive, and gradually recover when it stabilizes. Recommended for WiFi devices with weak signal.",
 
   "search.open": "Search (Ctrl+K)",
   "search.placeholder": "Search entities... (Ctrl+K)",
@@ -943,5 +946,25 @@
   "settings.restore.error": "Restore failed",
   "settings.restore.restarting": "Server is restarting...",
   "settings.restore.restart_timeout": "Server did not respond. Please refresh the page manually.",
-  "settings.button.close": "Close"
+  "settings.button.close": "Close",
+
+  "settings.auto_backup.label": "Auto-Backup",
+  "settings.auto_backup.hint": "Automatically create periodic backups of all configuration. Old backups are pruned when the maximum count is reached.",
+  "settings.auto_backup.enable": "Enable auto-backup",
+  "settings.auto_backup.interval_label": "Interval",
+  "settings.auto_backup.max_label": "Max backups",
+  "settings.auto_backup.save": "Save Settings",
+  "settings.auto_backup.saved": "Auto-backup settings saved",
+  "settings.auto_backup.save_error": "Failed to save auto-backup settings",
+  "settings.auto_backup.last_backup": "Last backup",
+  "settings.auto_backup.never": "Never",
+
+  "settings.saved_backups.label": "Saved Backups",
+  "settings.saved_backups.hint": "Auto-backup files stored on the server. Download to save locally, or delete to free space.",
+  "settings.saved_backups.empty": "No saved backups",
+  "settings.saved_backups.restore": "Restore",
+  "settings.saved_backups.download": "Download",
+  "settings.saved_backups.delete": "Delete",
+  "settings.saved_backups.delete_confirm": "Delete this backup file?",
+  "settings.saved_backups.delete_error": "Failed to delete backup"
 }