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/i18n.js

@@ -66,7 +66,7 @@ const LOCALES = {
     cmd_workspace: 'Switch workspace by name',
     cmd_new: 'Start a new chat session',
     cmd_usage: 'Toggle token usage display on/off',
-    cmd_theme: 'Switch theme (dark/light/slate/solarized/monokai/nord/oled)',
+    cmd_theme: 'Switch theme (system/dark/light/slate/solarized/monokai/nord/oled)',
     cmd_personality: 'Switch agent personality',
     cmd_skills: 'List available Hermes skills',
     available_commands: 'Available commands:',
@@ -480,7 +480,7 @@ const LOCALES = {
     cmd_workspace: 'Cambiar de espacio de trabajo por nombre',
     cmd_new: 'Iniciar una nueva sesión de chat',
     cmd_usage: 'Activar o desactivar el uso de tokens',
-    cmd_theme: 'Cambiar tema (dark/light/slate/solarized/monokai/nord/oled)',
+    cmd_theme: 'Cambiar tema (system/dark/light/slate/solarized/monokai/nord/oled)',
     cmd_personality: 'Cambiar la personalidad del agente',
     cmd_skills: 'Listar las skills de Hermes disponibles',
     available_commands: 'Comandos disponibles:',
@@ -884,7 +884,7 @@ const LOCALES = {
     cmd_workspace: 'Workspace nach Namen wechseln',
     cmd_new: 'Neue Chat-Sitzung starten',
     cmd_usage: 'Token-Verbrauchsanzeige umschalten',
-    cmd_theme: 'Theme wechseln (dark/light/slate/solarized/monokai/nord/oled)',
+    cmd_theme: 'Theme wechseln (system/dark/light/slate/solarized/monokai/nord/oled)',
     cmd_personality: 'Agenten-Persönlichkeit wechseln',
     cmd_skills: 'Verfügbare Hermes-Skills auflisten',
     available_commands: 'Verfügbare Befehle:',
@@ -1096,7 +1096,7 @@ const LOCALES = {
     cmd_workspace: '\u6309\u540d\u79f0\u5207\u6362\u5de5\u4f5c\u533a',
     cmd_new: '\u65b0\u5efa\u804a\u5929\u4f1a\u8bdd',
     cmd_usage: '\u5207\u6362 token \u7528\u91cf\u663e\u793a',
-    cmd_theme: '\u5207\u6362\u4e3b\u9898\uff08dark/light/slate/solarized/monokai/nord/oled\uff09',
+    cmd_theme: '\u5207\u6362\u4e3b\u9898\uff08system/dark/light/slate/solarized/monokai/nord/oled\uff09',
     cmd_personality: '\u5207\u6362 Agent \u4eba\u8bbe',
     cmd_skills: '\u5217\u51fa\u53ef\u7528\u7684 Hermes \u6280\u80fd',
     available_commands: '\u53ef\u7528\u547d\u4ee4\uff1a',
@@ -1499,7 +1499,7 @@ const LOCALES = {
     cmd_workspace: '\u6309\u540d\u7a31\u5207\u63db\u5de5\u4f5c\u5340',
     cmd_new: '\u65b0\u5efa\u804a\u5929\u6703\u8a71',
     cmd_usage: '\u5207\u63db token \u7528\u91cf\u986f\u793a',
-    cmd_theme: '\u5207\u63db\u4e3b\u984c\uff08dark/light/slate/solarized/monokai/nord/oled\uff09',
+    cmd_theme: '\u5207\u63db\u4e3b\u984c\uff08system/dark/light/slate/solarized/monokai/nord/oled\uff09',
     cmd_personality: '\u5207\u63db Agent \u4eba\u8a2d',
     cmd_skills: '\u5217\u51fa\u53ef\u7528\u7684 Hermes \u6280\u80fd',
     available_commands: '\u53ef\u7528\u547d\u4ee4\uff1a',