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>

packages/shared-auth-stores/package.json

@@ -0,0 +1,32 @@
+{
+  "name": "@manacore/shared-auth-stores",
+  "version": "0.0.1",
+  "type": "module",
+  "exports": {
+    ".": {
+      "types": "./src/index.ts",
+      "svelte": "./src/index.ts",
+      "default": "./src/index.ts"
+    }
+  },
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "svelte": "./src/index.ts",
+  "files": [
+    "src"
+  ],
+  "scripts": {
+    "type-check": "svelte-check --tsconfig ./tsconfig.json"
+  },
+  "peerDependencies": {
+    "svelte": "^5.0.0"
+  },
+  "devDependencies": {
+    "svelte": "^5.0.0",
+    "svelte-check": "^4.0.0",
+    "typescript": "^5.0.0"
+  },
+  "dependencies": {
+    "@manacore/shared-types": "workspace:*"
+  }
+}