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/index.html

@@ -4,7 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Hermes</title>
-<script>(function(){var t=localStorage.getItem('hermes-theme');if(t&&t!=='dark')document.documentElement.dataset.theme=t;})()</script>
+<script>(function(){var t=localStorage.getItem('hermes-theme');if(t==='system'){t=window.matchMedia('(prefers-color-scheme:dark)').matches?'dark':'light';}if(t&&t!=='dark')document.documentElement.dataset.theme=t;})()</script>
 <link rel="stylesheet" href="/static/style.css">
   <!-- KaTeX math rendering CSS (loaded eagerly to prevent layout shift) -->
   <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.22/dist/katex.min.css" integrity="sha384-5TcZemv2l/9On385z///+d7MSYlvIEw9FuZTIdZ14vJLqWphw7e7ZPuOiCHJcFCP" crossorigin="anonymous">
@@ -475,7 +475,8 @@
             </div>
             <div class="settings-field">
               <label for="settingsTheme" data-i18n="settings_label_theme">Theme</label>
-              <select id="settingsTheme" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px" onchange="document.documentElement.dataset.theme=this.value;localStorage.setItem('hermes-theme',this.value)">
+              <select id="settingsTheme" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px" onchange="_applyTheme(this.value)">
+                <option value="system">System (auto)</option>
                 <option value="dark">Dark (default)</option>
                 <option value="light">Light</option>
                 <option value="slate">Slate (charcoal)</option>