Support optional Clerk auth and add local preview mode

[maxwellt7]

Motivation

• Allow the dashboard to run in a local preview mode when Clerk environment variables are not set so the UI can build without external auth.
• Surface a clear notice on sign-in/sign-up pages and provide a fallback user UI when auth is not configured.
• Apply route protection only when Clerk is configured to avoid blocking local development.
• Centralize Clerk configuration detection for use across the app.

Description

• Added apps/dashboard/lib/clerk.ts and isClerkConfigured to detect presence of NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY and CLERK_SECRET_KEY and used it across the app.
• Made RootLayout render a plain AppShell when Clerk is not configured and otherwise wrap the app in ClerkProvider with signInUrl/signUpUrl set.
• Updated TabsLayout to only call auth() when Clerk is configured, default role to admin when not configured, and show a "Preview mode" badge in the header when running without Clerk.
• Replaced the previous global middleware.ts with proxy.ts that conditionally applies clerkMiddleware only if Clerk is configured and otherwise returns NextResponse.next().
• Added AuthConfigNotice component and updated sign-in and sign-up pages to display it when Clerk is not configured.
• Updated UserMenu to render a simple "Local preview" pill when Clerk is not configured instead of using UserButton.
• Added eslint.config.mjs and changed the lint script in apps/dashboard/package.json to run eslint with the new config followed by tsc --noEmit.
• Updated turbo.json to include Clerk environment variables in globalEnv so builds consider them.

Testing

• Ran lint and type checks via the new lint command (eslint . --config eslint.config.mjs --ext .js,.mjs and tsc --noEmit) and they completed successfully.
• Verified the app renders the sign-in/sign-up notice and header preview badge when NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY/CLERK_SECRET_KEY are absent, and that the clerkMiddleware path is exercised when those variables are present (manual dev verification).

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Codex Task