feat(web): home page is now the marketing site with auth-aware CTAs

- / now renders the full marketing page (was a silent auth redirect).
  Signed-in visitors see a "Dashboard →" CTA; signed-out visitors see
  "Sign in →". Both header nav + hero primary button flip based on a
  useAuthStatus() hook that calls /v1/me once on mount; "loading" state
  renders a dim placeholder so the wrong CTA never flashes.
- /welcome route removed (its content became /).
- signin page back-link now points at /.
- dashboard layout "About" link becomes "Home" -> /.
- footer links drop the dead sheetforge.dev pages in favour of in-page
  FAQ anchor + real GitHub repo link.
- header nav drops the 404-ing Docs/Pricing links and adds a FAQ anchor.

Behaviour: signed-in users landing on the domain get a product-forward
page with a dashboard button (no auto-redirect, they can still browse
the pitch), and signed-out users see the sign-in button as the primary
action.

Author: Devansh-365

apps/web/src/app/app/layout.tsx

@@ -123,11 +123,11 @@ export default function DashboardLayout({
               Projects
             </Link>
             <Link
-              href="/welcome"
+              href="/"
               className="transition-colors"
               style={{ color: "#b8b2b2" }}
             >
-              ← About
+              ← Home
             </Link>
             <span style={{ color: "#7f7a7a" }}>{user.email}</span>
             <button