feat: implement initial admin setup (Story 1.1)

Add complete initial admin setup functionality including:
- SetupForm with email, password, and password confirmation fields
- Password validation (minimum 12 characters) and confirmation matching
- Email format validation
- Setup route with GET (display form) and POST (process setup) handlers
- bcrypt password hashing before storing admin credentials
- Auto-login after successful setup with session management
- First-run detection middleware that redirects to /setup if no admin exists
- Setup page returns 404 after admin account is created
- Base HTML template with Pico CSS integration
- Admin dashboard placeholder template
- 404 error template

All tests pass with 90.09% code coverage (exceeds 80% requirement).
Code passes ruff linting and mypy type checking.

Story: 1.1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/templates/admin/dashboard.html

@@ -0,0 +1,15 @@
+{% extends "layouts/base.html" %}
+
+{% block title %}Admin Dashboard - Sneaky Klaus{% endblock %}
+
+{% block content %}
+<article>
+    <header>
+        <h1>Admin Dashboard</h1>
+    </header>
+
+    <p>Welcome to the Sneaky Klaus admin dashboard!</p>
+
+    <p>This is a placeholder for the admin dashboard. More features coming soon.</p>
+</article>
+{% endblock %}