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/base.html

@@ -27,6 +27,9 @@
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css"
           integrity="sha512-DTOQO9RWCH3ppGqcWaEA1BIZOC6xxalwEsw9c2QQeAIftl+Vegovlnee1c9QX4TctnWMn13TZye+giMm8e2LwA=="
           crossorigin="anonymous" referrerpolicy="no-referrer" />
+    <!-- flag-icons: SVG/CSS flag sprites that render correctly on all platforms including Windows -->
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flag-icons@7.3.2/css/flag-icons.min.css"
+          crossorigin="anonymous" referrerpolicy="no-referrer" />
     {% endblock %}
     {% block head_extra %}{% endblock %}
     <!-- CSRF token for AJAX/fetch requests -->
@@ -250,7 +253,7 @@
               >
                 <span class="text-base leading-none" aria-hidden="true">
                   {% set cur_lang = suggested_languages | selectattr("code", "equalto", current_locale) | list %}
-                  {% if cur_lang %}{{ cur_lang[0].flag }}{% else %}🌐{% endif %}
+                  {% if cur_lang %}<span class="fi fi-{{ cur_lang[0].flag }}"></span>{% else %}<i class="fas fa-globe"></i>{% endif %}
                 </span>
               </button>
               <div
@@ -290,7 +293,7 @@
                       class="flex items-center w-full px-4 py-2 text-sm text-left hover:bg-gray-100 dark:hover:bg-gray-700"
                       :onclick="`setLanguage('${lang.code}')`"
                     >
-                      <span class="mr-2 text-base" x-text="lang.flag"></span>
+                      <span class="fi mr-2" :class="`fi-${lang.flag}`" aria-hidden="true"></span>
                       <span x-text="lang.native"></span>
                       <i x-show="lang.code === '{{ current_locale }}'" class="fas fa-check ml-auto text-blue-500" aria-hidden="true"></i>
                     </button>