fix(ui): replace emoji flags with cross-platform flag-icons CSS library

Windows does not render regional indicator emoji as graphical flags,
showing raw text (DE, GB) instead. Replace all emoji flag values in
SUPPORTED_LANGUAGES with lowercase ISO 3166-1 alpha-2 country codes,
add the flag-icons@7.3.2 CSS library via CDN, and update templates to
render <span class="fi fi-{code}"> instead of emoji text.

Special cases:
- Welsh (cy): uses flag-icons region code "gb-wls"
- Catalan (ca): falls back to "es" (no dedicated ISO flag)
- Esperanto (eo): uses "un" (UN flag for international language)

<option> elements in profile.html no longer display flags since CSS
icon classes cannot be applied inside native <option> tags.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/profile.html

@@ -179,7 +179,7 @@
         >
           <option value="">{{ _("profile.language_auto_detect") }}</option>
           {% for lang in supported_languages %}
-          <option value="{{ lang.code }}">{{ lang.flag }} {{ lang.native }} ({{ lang.name }})</option>
+          <option value="{{ lang.code }}">{{ lang.native }} ({{ lang.name }})</option>
           {% endfor %}
         </select>
         <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
@@ -246,7 +246,7 @@
         >
           <option value="">{{ _("profile.default_document_language_auto") }}</option>
           {% for lang in supported_languages %}
-          <option value="{{ lang.code }}">{{ lang.flag }} {{ lang.native }} ({{ lang.name }})</option>
+          <option value="{{ lang.code }}">{{ lang.native }} ({{ lang.name }})</option>
           {% endfor %}
         </select>
         <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">