feat: donation banner, About tab, settings UI improvements

- Dismissible donation/open-source banner after 3+ sessions (30-day snooze)
- New About tab in Settings: version, repo link, license info
- Centralize project URLs (REPO_URL, DONATE_URL) in __init__.py, served via /health
- Center settings tab bar, reduce tab padding for 6-tab fit
- External URL save button: icon button instead of full-width text button
- Remove redundant settings footer close button
- Footer "Source Code" link replaced with "About" opening settings
- i18n keys for en/ru/zh

server/src/wled_controller/templates/index.html

@@ -105,6 +105,7 @@
             </div>
         </header>
         <div id="update-banner" class="update-banner" style="display:none"></div>
+        <div id="donation-banner" class="donation-banner" style="display:none"></div>
     <div class="container">
         <div class="tabs">
             <div class="tab-panel" id="tab-dashboard" role="tabpanel" aria-labelledby="tab-btn-dashboard">
@@ -170,7 +171,7 @@
                 <p>
                     Created by <strong>Alexei Dolgolyov</strong>
                     • <a href="mailto:dolgolyov.alexei@gmail.com">dolgolyov.alexei@gmail.com</a>
-                    • <a href="https://git.dolgolyov-family.by/alexei.dolgolyov/wled-screen-controller-mixed" target="_blank" rel="noopener">Source Code</a>
+                    • <a href="#" onclick="openSettingsModal();switchSettingsTab('about');return false" data-i18n="donation.about_title">About</a>
                 </p>
             </div>
         </footer>