feat: add Tier 3 shared auth store patterns

- Create @manacore/shared-auth-stores package with Svelte 5 factories:
  - createAuthStore<T>(): Generic factory with custom auth service adapter
  - createSupabaseAuthStore(): Direct Supabase integration for simpler setups
- Add standardized auth error types to @manacore/shared-types:
  - AuthErrorCode enum for consistent error handling
  - mapSupabaseErrorToCode() helper function
  - createAuthError() factory function

This enables apps to share auth state management while supporting
both middleware-based auth (ManaCore/Manadeck) and direct Supabase
auth (ManaCore-web simple mode).

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

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

pnpm-lock.yaml

@@ -1576,6 +1576,22 @@ importers:
         specifier: ^5.9.3
         version: 5.9.3
 
+  packages/shared-auth-stores:
+    dependencies:
+      '@manacore/shared-types':
+        specifier: workspace:*
+        version: link:../shared-types
+    devDependencies:
+      svelte:
+        specifier: ^5.0.0
+        version: 5.43.14
+      svelte-check:
+        specifier: ^4.0.0
+        version: 4.3.4(picomatch@4.0.3)(svelte@5.43.14)(typescript@5.9.3)
+      typescript:
+        specifier: ^5.0.0
+        version: 5.9.3
+
   packages/shared-auth-ui:
     dependencies:
       '@manacore/shared-auth':