Upgrade: Match End Lock + Winner Congratulations + Next Match Overlay

Added:
1. Completed match lock
   - Completed matches cannot be resumed or started again.
   - Match list shows a disabled Completed button.
   - Start match dropdown hides completed matches.

2. Winner full-screen congratulations
   - End Match now calculates winner/result.
   - Overlay plays transparent full-screen Congratulations animation.
   - Uses existing transparent animation layer, no replay scene added.

3. Next match overlay
   - Overlay shows the next scheduled/live match during the current match.
   - Next match card is transparent/glass style at top-right.

Changed files:
- api/end_match.php
- api/get_score.php
- api/tournament_match_crud.php
- overlay.php
- assets/js/overlay.js
- assets/js/tournament.js
- assets/css/overlay.css
- assets/css/dashboard.css