feat(ui): replace death screen space bar prompt with tappable restart button

[f1cklepickle]

Why

Death screen restart was spacebar-only and inaccessible on Android/touch devices.
We need one restart flow that works for both touch and keyboard inputs.

What changed

• Added a dedicated game-over UI controller to own restart button wiring and presentation
• Kept Space restart behavior while adding tappable restart support
• Updated death-screen messaging to touch-friendly copy
• Added EditMode and PlayMode regression coverage for restart behavior
• Logged coverage update in Docs/TEST_LOG.md

How to test

1. Open MainPrototype
2. Press Play, die, then verify a tappable restart button appears and restarts the scene
3. Press Space on game-over and verify restart still works
4. Run tests:
   • EditMode (GameOverRestartContractsTests, GameManagerKeyboardRestartContractsTests)
   • PlayMode (DeathScreenRestartPlayTests)

Checklist

All items must be checked. If not applicable, check and mark "N/A".

• Unit tests (EditMode) added or updated
• PlayMode test or manual validation included
• Demo scene updated (if player-visible)
• Prefab links, tags, and layers validated N/A
• README / Docs touched (if applicable)

Related

• Closes feat(ui): replace death screen space bar prompt with tappable restart button #139