docs: add guard concept page

[Copilot]

No conceptual overview existed explaining guards as a unified system — how they work, where to use them, and how to build a reusable guard library.

Added docs/concepts/guard.md

• Core concepts — the three guard results (pass, hidden, redirectTo), sequential evaluation, and short-circuit behavior
• Usage locations — route guards (defineModule/defineResource) vs component guards (WithGuard), and what each affects (sidebar visibility, URL access, DOM rendering)
• Context data — typing contextData via AppShellRegister for fully typed guard functions
• Common patterns — auth checks, RBAC, permission strings, feature flags, async API checks, tenant tiers
• Reusable guard library — how to define guards once in a shared guards.ts and reuse across both routes and components
• Comparison table — route guards vs WithGuard across key dimensions (redirect support, navigation hiding, URL blocking, async)
• Cross-links — to modules-and-resources.md, with-guard.md, and the guards API reference