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/panels.js

@@ -1112,7 +1112,7 @@ function toggleSettings(){
   if(!overlay) return;
   if(overlay.style.display==='none'){
     _settingsDirty = false;
-    _settingsThemeOnOpen = document.documentElement.dataset.theme || 'dark';
+    _settingsThemeOnOpen = localStorage.getItem('hermes-theme') || document.documentElement.dataset.theme || 'dark';
     _settingsSection = 'conversation';
     overlay.style.display='';
     loadSettingsPanel();
@@ -1150,8 +1150,9 @@ function _closeSettingsPanel(){
 // Revert live DOM/localStorage to what they were when the panel opened
 function _revertSettingsPreview(){
   if(_settingsThemeOnOpen){
-    document.documentElement.dataset.theme = _settingsThemeOnOpen;
     localStorage.setItem('hermes-theme', _settingsThemeOnOpen);
+    if(typeof _applyTheme==='function') _applyTheme(_settingsThemeOnOpen);
+    else document.documentElement.dataset.theme = _settingsThemeOnOpen;
   }
 }