feat: add Google OAuth, user profiles, community hub improvements

- Add Google OAuth sign-in alongside magic link authentication
- Username onboarding: require username selection after first sign-in
- Public profile pages at /u/[username] with social links (GitHub, X)
- Settings page for managing username and social links
- Viewer header: unified floating card with project name, @username, breadcrumb
- Guest CTA on viewer page prompting sign-in
- Show owner avatar + username on community project cards
- Redesigned project cards (larger preview, avatar + stats row below)
- Consistent navbar across hub and profile pages with GitHub link
- Footer with links to GitHub repo and npm packages
- Borderless design with natural shadows throughout
- Apple corner smoothing (squircle) progressive enhancement
- cursor:pointer globally on buttons and links
- DB migrations: username, github_url, x_url columns on auth_users
- Root-level db:generate/migrate/push/studio scripts

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

packages/auth/src/server.ts

@@ -16,6 +16,10 @@ export interface AuthConfig {
   appName: string
   baseURL: string
   secret: string
+  /** Google OAuth client ID */
+  googleClientId?: string
+  /** Google OAuth client secret */
+  googleClientSecret?: string
   /** Callback to send magic link emails */
   sendMagicLink?: (params: SendMagicLinkParams) => Promise<void>
   /** Additional plugins to add (e.g., nextCookies for web) */
@@ -57,6 +61,22 @@ export function createAuth(config: AuthConfig): ReturnType<typeof betterAuth> {
         },
       },
     },
+    // Google OAuth provider (only enabled when credentials are provided)
+    ...(config.googleClientId &&
+      config.googleClientSecret && {
+        socialProviders: {
+          google: {
+            clientId: config.googleClientId,
+            clientSecret: config.googleClientSecret,
+          },
+        },
+        account: {
+          accountLinking: {
+            enabled: true,
+            trustedProviders: ['google'],
+          },
+        },
+      }),
     plugins: [
       ...(config.additionalPlugins ?? []),
       // Magic link authentication