test(desktop): pin the clock in the session-row timestamp test

The sidebar row's Tip trigger test derives its fixture from the wall clock:

    const startedAt = Math.floor(Date.now() / 1000) - 5 * 60
    ...
    expect(age.getAttribute('aria-label')).toMatch(/^5m, Today at /)

"Five minutes ago" is only today when the run does not straddle local
midnight. Between 00:00 and 00:05 the timestamp falls into the previous
day, formatMessageTimestamp correctly returns the yesterday label, and
the test fails on a day boundary it was never written to exercise:

    AssertionError: expected '5m, Yesterday at 11:56 PM'
                    to match /^5m, Today at /

This is a real CI failure, not a theoretical one - it took down a
check:test:ui shard on an unrelated desktop PR at 00:01 UTC, and it will
do so for any PR whose shard happens to land in that five-minute window.

Pin the clock to local noon before deriving the timestamp so the fixture
can never cross a day boundary. Only Date is faked (toFake: ['Date']),
so the component's own timers - the running arc and the tooltip open
delay - keep running for real; the neighbouring tooltip tests that
advance timers are unaffected. The describe block gains the
useRealTimers teardown its sibling already had.

The production formatter is not changed: rendering "Yesterday at 11:56
PM" for a session started five minutes before midnight is correct, and
the assertion is about the label's composition, not about which day it
names.

apps/desktop/src/app/chat/sidebar/session-row.test.tsx

@@ -231,6 +231,10 @@ describe('SidebarSessionRow running arc', () => {
 })
 
 describe('SidebarSessionRow', () => {
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
   it('keeps an aria-label on the kebab without wrapping it in a Tip', () => {
     render(
       <SidebarSessionRow
@@ -322,6 +326,17 @@ describe('SidebarSessionRow', () => {
   })
 
   it('exposes the exact session time through a focusable Tip trigger', () => {
+    // Pin the clock before deriving the timestamp.  The assertion below is
+    // about the *composition* of the label (relative age + absolute time),
+    // but "5 minutes ago" only falls on today when the run does not straddle
+    // local midnight.  Between 00:00 and 00:05 the row correctly renders
+    // "Yesterday at 11:5x PM" and this test failed for a day boundary it was
+    // never written to exercise.  Only `Date` is faked, so the component's
+    // own timers (the running arc, the tooltip open delay) keep running for
+    // real.
+    vi.useFakeTimers({ toFake: ['Date'] })
+    vi.setSystemTime(new Date(2026, 2, 5, 12, 0, 0))
+
     const startedAt = Math.floor(Date.now() / 1000) - 5 * 60
 
     render(