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>

composer.json

@@ -22,9 +22,10 @@
         "mintyphp/tools": "*",
         "mintyphp/debugger": "*",
         "phpunit/phpunit": "*",
-        "phpstan/phpstan": "^1.9",
+        "phpstan/phpstan": "^2.1",
         "icanhazstring/composer-unused": "^0.9.6",
-        "friendsofphp/php-cs-fixer": "^3.66"
+        "friendsofphp/php-cs-fixer": "^3.66",
+        "tomasvotruba/unused-public": "^2.2"
     },
     "autoload-dev": {
         "psr-4": {