fix: wrap GameOverScreen in ErrorBoundary and add defensive checks for day 30 blank page bug

- Wrap GameOverScreen in ErrorBoundary in page.tsx to prevent blank page on render errors
- Add defensive Number.isFinite checks in GameOverScreen for all numeric props
- Add regression test for day 30 → game-over flow (day30-blank-page.test.ts)

Fixes #375

docs/project-structure.txt

@@ -217,6 +217,7 @@ Mana-Loop/
 │   │   │   │   ├── cross-module-lifecycle-consistency.test.ts
 │   │   │   │   ├── cross-module-prestige-discipline.test.ts
 │   │   │   │   ├── curse-amplification.test.ts
+│   │   │   │   ├── day30-blank-page.test.ts
 │   │   │   │   ├── design-validation-perk-gating.test.ts
 │   │   │   │   ├── discipline-deactivate-on-spire-entry.test.ts
 │   │   │   │   ├── discipline-math.test.ts