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

TODO-css-improvements.md

@@ -13,10 +13,11 @@
 
 ## Donation / Open-Source Banner
 
-- [ ] Add a persistent but dismissible banner or notification in the dashboard UI informing users that the project is open-source and under active development, and that donations are highly appreciated
-- [ ] Include a link to the donation page (GitHub Sponsors, Ko-fi, or similar — decide on platform)
-- [ ] Remember dismissal in localStorage so it doesn't reappear every session
-- [ ] Add i18n keys for the banner text (`en.json`, `ru.json`, `zh.json`)
+- [x] Add a persistent but dismissible banner or notification in the dashboard UI informing users that the project is open-source and under active development, and that donations are highly appreciated
+- [x] Include a link to the donation page (GitHub Sponsors, Ko-fi, or similar — decide on platform)
+- [x] Remember dismissal in localStorage so it doesn't reappear every session
+- [x] Add i18n keys for the banner text (`en.json`, `ru.json`, `zh.json`)
+- [ ] Configure `DONATE_URL` and `REPO_URL` constants in `donation.ts` once platform is chosen
 
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