Security: rate limiting and flash messaging for partage share links

- Add rate limiting (5 submissions per IP per 10 min, per share link)
  to prevent abuse of shared submission endpoints
- Replace all plain die() error responses with styled flash messages
  and redirects (invalid slug, disabled link, expired link, wrong password,
  rate limit exceeded, CSRF failure)
- Add dedicated error page renderer for disabled/expired links with
  home page link
- Password gate now uses flash message via session redirect instead
  of inline error variable

TODO.md

@@ -28,6 +28,6 @@
   - [x] Implement delete link action
   - [x] Copy-to-clipboard button for full partage URL
 
-- [ ] Security and validation considerations
-  - [ ] Rate limiting on form submissions per share link
-  - [ ] Add flash messages / error handling for invalid/disabled/password-protected links
+- [x] Security and validation considerations
+  - [x] Rate limiting on form submissions per share link — integrate RateLimit into partage index.php POST handler
+  - [x] Add flash messages / error handling for invalid/disabled/password-protected links — replace plain die() with styled error pages and flash messages