feat(navigation): persist last-port for next-login + root → /dashboard

Login routing previously always landed at the user's first port-role.
With a multi-port operator (super-admins, multi-tenant ops) the active
port reverted on every login, breaking the "I was working in X
yesterday" continuity.

- PortProvider PATCHes `/api/v1/me` with `preferences.defaultPortId =
  currentPort.id` whenever the active port changes (URL or explicit
  switch). Ref-keyed dedupe; fire-and-forget so navigation isn't
  blocked by a transient PATCH failure.
- UserMenu's port-switcher also writes the preference on click so the
  preference is captured even for users who never re-render through
  PortProvider.
- /dashboard resolver checks `preferences.defaultPortId` first, falling
  back to first-port-by-name (super-admin) or first-role (everyone
  else). The preference is verified against current access before being
  honoured — a stale id from a revoked role or archived port can't
  strand the user on a 403.
- Add /src/app/page.tsx that redirects `/` → `/dashboard` so the
  middleware's `redirect=/` post-login parameter doesn't dump users on
  an empty 404. The existing /dashboard handler then routes them on to
  their resolved port.
- UserMenu sign-out: replace `router.push('/api/auth/sign-out')` (which
  issued a GET against better-auth's POST-only endpoint, causing Safari
  and Comet/Arc to land the JSON response as a `sign_out` download)
  with `signOut()` from the auth client + an explicit redirect.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/layout/user-menu.tsx

@@ -19,6 +19,8 @@ import { LogOut, Settings, Bell, Check, Building2 } from 'lucide-react';
 import { type ReactNode } from 'react';
 
 import { useUIStore } from '@/stores/ui-store';
+import { signOut } from '@/lib/auth/client';
+import { apiFetch } from '@/lib/api/client';
 import {
   DropdownMenu,
   DropdownMenuContent,
@@ -61,6 +63,16 @@ export function UserMenu({ trigger, align = 'end', user, ports }: UserMenuProps)
     // All cached queries are port-scoped - invalidate so they refetch with
     // the new X-Port-Id header.
     queryClient.invalidateQueries();
+    // Remember the choice so the next login lands here automatically.
+    // Fire-and-forget — failure shouldn't block the navigation, and any
+    // stale value is harmless (the post-login resolver verifies access
+    // before honouring it).
+    void apiFetch('/api/v1/me', {
+      method: 'PATCH',
+      body: { preferences: { defaultPortId: port.id } },
+    }).catch(() => {
+      /* silent — best-effort */
+    });
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     router.push(`/${port.slug}/dashboard` as any);
   }
@@ -122,7 +134,16 @@ export function UserMenu({ trigger, align = 'end', user, ports }: UserMenuProps)
         <DropdownMenuSeparator />
         <DropdownMenuItem
           className="text-destructive focus:text-destructive"
-          onClick={() => router.push('/api/auth/sign-out')}
+          // `router.push` to /api/auth/sign-out issued a GET against
+          // better-auth's POST-only endpoint. Safari (and other browsers
+          // serving JSON without a matching renderer) treat that as a
+          // file download — the response landed on disk as a `sign_out`
+          // file instead of signing the user out. Call the auth client
+          // directly to issue a proper POST, then redirect to /login.
+          onClick={async () => {
+            await signOut();
+            router.push('/login');
+          }}
         >
           <LogOut className="w-4 h-4 mr-2" aria-hidden />
           Sign Out