feat: pluggable i18n with English/Chinese language switcher in Settings

Introduces a locale bundle system that makes UI language switchable at
runtime and trivially extensible to any future language.

Architecture:
- static/i18n.js: LOCALES object with 'en' and 'zh' bundles, t(key)
  helper with English fallback, setLocale()/loadLocale() for persistence
  via localStorage. Adding a new language = adding one object.
- api/config.py: 'language' setting (default 'en'), BCP-47 validation
- api/routes.py: _LOGIN_LOCALE dict for server-rendered login page;
  template placeholders substituted at request time from saved setting
- static/index.html: loads i18n.js first (before other scripts); adds
  Language dropdown to Settings panel, auto-populated from LOCALES

Wiring:
- boot.js: applies server-persisted locale at startup (after /api/settings
  fetch); speech recognition lang follows _locale._speech
- panels.js: populates Language dropdown from LOCALES on settings open;
  saves + applies locale on Save Settings
- All JS files: hardcoded user-facing strings replaced with t() calls

Coverage:
- test_sprint20.py: relaxed recognition.lang assertion to accept dynamic
  locale-driven assignment (behavior unchanged for English default)
- 499/499 tests pass

Closes #177 (incorporates Chinese translations as a proper locale bundle
rather than hardcoded strings, so English default is fully preserved)

static/index.html

@@ -345,6 +345,10 @@
           <option value="oled">OLED</option>
         </select>
       </div>
+      <div class="settings-field">
+        <label for="settingsLanguage">Language</label>
+        <select id="settingsLanguage" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px"></select>
+      </div>
       <div class="settings-field">
         <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
           <input type="checkbox" id="settingsSoundEnabled" style="width:15px;height:15px;accent-color:var(--accent)">
@@ -427,6 +431,7 @@
   </button>
 </nav>
 <div class="toast" id="toast"></div>
+<script src="/static/i18n.js"></script>
 <script src="/static/ui.js"></script>
 <script src="/static/workspace.js"></script>
 <script src="/static/sessions.js"></script>