feat: add System (auto) theme following OS prefers-color-scheme (#504)

Synthesized from PRs #506, #509, #514 (all by armorbreak001 and cloudyun888).

Implementation:
- static/index.html: flicker-prevention head script resolves 'system' to
  'dark'/'light' via matchMedia before first paint. Adds 'System (auto)'
  as first option in theme picker. onchange calls _applyTheme().
- static/boot.js: new _applyTheme(name) helper — resolves 'system' via
  matchMedia, sets data-theme, registers a MQ change listener so the UI
  tracks OS switches live. loadSettings() now calls _applyTheme() instead
  of direct data-theme assignment.
- static/commands.js: adds 'system' to valid /theme command names,
  delegates apply to _applyTheme().
- static/panels.js: _settingsThemeOnOpen reads from localStorage (preserves
  'system' string, not the resolved 'dark'/'light'). _revertSettingsPreview
  calls _applyTheme() so reverting to 'system' correctly re-enables OS tracking.
- static/i18n.js: cmd_theme description now lists 'system' first in all 5
  locales (en, es, de, zh-Hans, zh-Hant).

Design choices vs submitted PRs:
- No separate system-theme.js file (unnecessary indirection).
- matchMedia listener does NOT POST to /api/settings (OS can change rapidly;
  persisting on every OS switch would hammer the server).

Co-authored-by: armorbreak001 <armorbreak001@users.noreply.github.com>
Co-authored-by: cloudyun888 <cloudyun888@users.noreply.github.com>

static/boot.js

@@ -568,6 +568,21 @@ window.addEventListener('resize',()=>{
   };
 })();
 
+// ── System theme helper ──────────────────────────────────────────────────────
+function _applyTheme(name){
+  const resolved=(name==='system')
+    ?(window.matchMedia('(prefers-color-scheme:dark)').matches?'dark':'light')
+    :name;
+  document.documentElement.dataset.theme=resolved||'dark';
+  // Re-register OS change listener whenever system theme is active
+  if(name==='system'){
+    const mq=window.matchMedia('(prefers-color-scheme:dark)');
+    const _onOsChange=()=>{ document.documentElement.dataset.theme=mq.matches?'dark':'light'; };
+    mq.removeEventListener('change',_onOsChange);
+    mq.addEventListener('change',_onOsChange);
+  }
+}
+
 function applyBotName(){
   const name=window._botName||'Hermes';
   document.title=name;
@@ -594,8 +609,8 @@ function applyBotName(){
     window._notificationsEnabled=!!s.notifications_enabled;
     window._botName=s.bot_name||'Hermes';
     const _theme=s.theme||'dark';
-    document.documentElement.dataset.theme=_theme;
     localStorage.setItem('hermes-theme',_theme);
+    _applyTheme(_theme);
     document.body.classList.toggle('bubble-layout',!!s.bubble_layout);
     if(typeof setLocale==='function'){
       const _lang=typeof resolvePreferredLocale==='function'