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)

tests/test_sprint20.py

@@ -213,9 +213,14 @@ def test_boot_js_recognition_interim_results():
 
 
 def test_boot_js_recognition_lang_en():
-    """recognition.lang must be set to en-US."""
+    """recognition.lang must be set (static en-US or dynamic via _locale._speech)."""
     js, _ = get_text("/static/boot.js")
-    assert "recognition.lang='en-US'" in js or 'recognition.lang = "en-US"' in js or "recognition.lang='en-US'" in js
+    # Accept either the old static value or the new locale-driven assignment
+    assert (
+        "recognition.lang='en-US'" in js
+        or 'recognition.lang = "en-US"' in js
+        or "recognition.lang=" in js  # dynamic: recognition.lang=(_locale._speech)||'en-US'
+    )
 
 
 def test_boot_js_onresult_handler():