fix(dashboard,auth): today-window on @db.Date + query-cache cleared across logins

Two dashboard bugs reported after clock-in:

1. 'Dochazka dnes' / 'Pritomni dnes' showed everyone absent even after
   refresh: attendance.shift_date is @db.Date and Prisma truncates filter
   Dates to their UTC date part, so the local-midnight boundaries
   (new Date(y,m,d) = 22:00/23:00Z of the previous day) queried
   [yesterday, today) and matched zero of today's punches. Boundaries are
   now UTC-midnight instants of the local (Prague) calendar day.
   Reproduced empirically against real rows; route-level regression test
   added (dashboard.test.ts).

2. After logout + login as a different user, cached dashboards/buttons from
   the previous user were served: query keys are user-agnostic and the
   React Query cache outlives the session. logout(), login() and the 2FA
   verify path now clear the whole cache.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/routes/admin/dashboard.ts

@@ -11,15 +11,16 @@ export default async function dashboardRoutes(
 ): Promise<void> {
   fastify.get("/", { preHandler: requireAuth }, async (request, reply) => {
     const now = new Date();
+    // shift_date is @db.Date: Prisma truncates a filter Date to its UTC date
+    // part, so these boundaries MUST be UTC-midnight instants of the LOCAL
+    // (Prague) calendar day. A local-midnight Date (new Date(y, m, d)) is
+    // 22:00/23:00 UTC of the PREVIOUS day and silently shifts the window to
+    // [yesterday, today) — the "Docházka dnes" card then matches zero rows.
     const todayStart = new Date(
-      now.getFullYear(),
-      now.getMonth(),
-      now.getDate(),
+      Date.UTC(now.getFullYear(), now.getMonth(), now.getDate()),
     );
     const todayEnd = new Date(
-      now.getFullYear(),
-      now.getMonth(),
-      now.getDate() + 1,
+      Date.UTC(now.getFullYear(), now.getMonth(), now.getDate() + 1),
     );
     const monthStart = new Date(now.getFullYear(), now.getMonth(), 1);
     const monthEnd = new Date(now.getFullYear(), now.getMonth() + 1, 1);