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/static/locales/en.json

@@ -1940,6 +1940,7 @@
     "appearance.bg.applied": "Background effect applied",
 
     "settings.tab.updates": "Updates",
+    "settings.tab.about": "About",
     "update.status_label": "Update Status",
     "update.current_version": "Current version:",
     "update.badge_tooltip": "New version available — click for details",
@@ -2009,5 +2010,15 @@
     "asset.type.video": "Video",
     "asset.type.other": "Other",
     "streams.group.assets": "Assets",
-    "section.empty.assets": "No assets yet. Click + to upload one."
+    "section.empty.assets": "No assets yet. Click + to upload one.",
+
+    "donation.message": "LedGrab is free & open-source. If it's useful to you, consider supporting development.",
+    "donation.support": "Support the project",
+    "donation.view_source": "View source code",
+    "donation.later": "Remind me later",
+    "donation.dismiss": "Don't show again",
+    "donation.about_title": "About LedGrab",
+    "donation.about_opensource": "LedGrab is open-source software, free to use and modify.",
+    "donation.about_donate": "Support development",
+    "donation.about_license": "MIT License"
 }