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/js/app.js

@@ -137,6 +137,7 @@ import { navigateToCard } from './core/navigation.js';
 import { openCommandPalette, closeCommandPalette, initCommandPalette } from './core/command-palette.js';
 import {
     openSettingsModal, closeSettingsModal, downloadBackup, handleRestoreFileSelected,
+    saveAutoBackupSettings, restoreSavedBackup, downloadSavedBackup, deleteSavedBackup,
 } from './features/settings.js';
 
 // ─── Register all HTML onclick / onchange / onfocus globals ───
@@ -388,11 +389,15 @@ Object.assign(window, {
     openCommandPalette,
     closeCommandPalette,
 
-    // settings (backup / restore)
+    // settings (backup / restore / auto-backup)
     openSettingsModal,
     closeSettingsModal,
     downloadBackup,
     handleRestoreFileSelected,
+    saveAutoBackupSettings,
+    restoreSavedBackup,
+    downloadSavedBackup,
+    deleteSavedBackup,
 });
 
 // ─── Global keyboard shortcuts ───