Fix Eloquent user IDs and update MCP dependency

[sylvesterdamgaard]

What changed

• Normalize Statamic user IDs before storing or filtering MCP tokens so Eloquent integer IDs satisfy strict string contracts.
• Apply the same normalization in the OAuth authorization flow.
• Widen the laravel/mcp dependency to support ^0.6 and ^0.7, and update docs/install guidance to match.
• Add regression tests for integer Eloquent IDs, string IDs, and missing current users.

Why

Sites using Statamic Eloquent users can return integer user IDs, which caused a TypeError in the MCP dashboard under strict types. Fresh installs can also be blocked by the older laravel/mcp constraint when using newer MCP packages.

Fixes #31.

Verification

• composer validate --strict
• vendor/bin/pint --test tests/Unit/Http/Controllers/CP/ResolvesUserIdTest.php
• vendor/bin/pest tests/Unit/Support/UserIdsTest.php tests/Unit/Http/Controllers/CP/ResolvesUserIdTest.php tests/E2E/UserDashboardTest.php tests/E2E/TokenLifecycleTest.php