test(main-page): drop stale value-proposition assertions

[uuzzrm]

What & why

48267e5 ("Remove unnecessary headline") removed the empty-state headline from MainPage.tsx:

-        {isQueryEmpty && (
-          <Text ta="center" size="sm">
-            Private AI search: answers with sources, on your own instance.
-          </Text>
-        )}

but left the two tests that assert that text behind. The positive one ("shows the value proposition in the empty state") has been failing consistently since — locally and in CI (build-test on main is red for it) — and the negative one now tests a feature that no longer exists, so it passes only because the text is never rendered.

This removes both stale test cases.

Test plan

• npx vitest run client/components/Pages/Main/MainPage.test.tsx → 12 pass (was 13 pass + 1 fail)
• Full npx vitest run → no new failures (the only 2 remaining failures locally are the doc-validator/doc-gardening script tests, which fail on this Windows checkout regardless of any change — they pass on CI)

Nothing else references the removed text.

[uuzzrm]

Hey - quick note that #2388 (identical change, same two test deletions) was merged a few minutes ago, so this PR is now redundant. Closing to keep the queue tidy. Thanks for the fast merge!

[uuzzrm]

Scratch that - I misread the merge state. #2388 was the duplicate that got closed; this PR is the canonical one and it is still green. Reopening. The stale MainPage assertions removal is correct here and unblocks #2385's CI once merged.