fix: Wire up VDL2 agent mode, fix dashboard layout gap and stats strip sizing

- Add VDL2 to syncModeUI setter map and allModes array in agents.js
  so agent state sync works for VDL2
- Fix dashboard bottom gap by using flex layout on body instead of
  hardcoded calc(100dvh - 160px) height
- Match source stat font-size to other stats (14px) for consistent
  strip sizing
- Add left-sidebars wrapper, VDL2 agent mode support, mutual sidebar
  collapse, and ACARS/VDL2 modeNames in index.html

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

templates/index.html

@@ -3700,7 +3700,8 @@
                 'pager': 'pager', 'sensor': '433',
                 'satellite': 'satellite', 'wifi': 'wifi', 'bluetooth': 'bluetooth', 'bt_locate': 'bt locate',
                 'listening': 'listening', 'aprs': 'aprs', 'tscm': 'tscm', 'meshtastic': 'meshtastic',
-                'dmr': 'dmr', 'websdr': 'websdr', 'sstv_general': 'hf sstv'
+                'dmr': 'dmr', 'websdr': 'websdr', 'sstv_general': 'hf sstv',
+                'acars': 'acars', 'vdl2': 'vdl2'
             };
             document.querySelectorAll('.mode-nav-btn').forEach(btn => {
                 const label = btn.querySelector('.nav-label');