refactor(auth): absorb shared-auth-stores into shared-auth-ui

Merge the auth store factories (createManaAuthStore, createAuthStore) from
@manacore/shared-auth-stores into @manacore/shared-auth-ui, reducing
from 3 auth packages to 2.

- Copy store files into shared-auth-ui/src/stores/
- Re-export store factories and types from shared-auth-ui
- Update imports in manacore/web and arcade/web
- Remove shared-auth-stores from active package.json dependencies

Result: @manacore/shared-auth (core, platform-agnostic) +
        @manacore/shared-auth-ui (Svelte components + stores)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/manacore/apps/web/package.json

@@ -50,7 +50,6 @@
 		"@manacore/local-store": "workspace:*",
 		"@manacore/qr-export": "workspace:*",
 		"@manacore/shared-auth": "workspace:*",
-		"@manacore/shared-auth-stores": "workspace:*",
 		"@manacore/shared-auth-ui": "workspace:*",
 		"@manacore/shared-branding": "workspace:*",
 		"@manacore/shared-config": "workspace:*",

apps/manacore/apps/web/src/lib/stores/auth.svelte.ts

@@ -2,6 +2,6 @@
  * Auth Store — uses centralized Mana auth factory.
  */
 
-import { createManaAuthStore } from '@manacore/shared-auth-stores';
+import { createManaAuthStore } from '@manacore/shared-auth-ui';
 
 export const authStore = createManaAuthStore();