feat: account linking for magic link + Google, last login method hint

- Move account linking config out of Google conditional so it's always active
- Trust both 'google' and 'email' providers for automatic account linking
- Add lastLoginMethod plugin to track how users last signed in
- Show "Last signed in with Google/email link" hint in sign-in dialog

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

packages/auth/src/client.ts

@@ -1,4 +1,4 @@
-import { magicLinkClient } from 'better-auth/client/plugins'
+import { lastLoginMethodClient, magicLinkClient } from 'better-auth/client/plugins'
 import { createAuthClient } from 'better-auth/react'
 
 /**
@@ -45,7 +45,7 @@ function getAuthURL(): string {
  */
 export const authClient = createAuthClient({
   baseURL: getAuthURL(),
-  plugins: [magicLinkClient()],
+  plugins: [magicLinkClient(), lastLoginMethodClient()],
 })
 
 /**