chore: upgrade PHPStan to 2.x and add unused-public dead code detection

- Upgrade phpstan/phpstan 1.12 → 2.1
- Add tomasvotruba/unused-public 2.2 for automatic dead code detection
- Generate baseline (486 existing findings) to keep CI green
- Remove stale @phpstan-ignore annotation in login page (ternary.alwaysFalse)
- New code is fully checked against PHPStan 2 + unused-public rules

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

phpstan.neon

@@ -1,3 +1,7 @@
+includes:
+    - vendor/tomasvotruba/unused-public/config/extension.neon
+    - phpstan-baseline.neon
+
 parameters:
     level: 5
     phpVersion: 80499