fix(app): restore history loading after session switch

[ytfh44]

When switching back to a session whose message cache was evicted, only 2 messages were loaded (initialMessagePageSize). If those filled the viewport, fill() would bail out and never trigger loadOlder(), leaving the user stuck with no visible history.

Two fixes: fill() no longer skips loadOlder when message count is <= 2, even if the viewport is full. Added a scroll-to-top listener that triggers loadOlder when the user scrolls near the top (scrollTop < 200px), matching the existing onHistoryScroll continuation threshold.

Issue for this PR

Closes #34187

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