[BUG] Refresh scroll indicators after switching usage views #44

Describe the bug
The /usage dialog keeps stale more above and/or more below indicators after switching between content views. This affects regular usage, trends, and session views. The indicators can remain visible even when the newly displayed content does not scroll.

To Reproduce

1. Open /usage.
2. Display a period with enough content to produce scroll indicators, or scroll within the dialog.
3. Switch to trends with g, session view with s, or another usage mode/period with m or the arrow keys.
4. Observe the scroll indicators for the newly displayed content.

Expected behavior
Scroll position and scrollability should be recalculated from the content currently rendered in the scrollbox after every view, mode, period, and data transition. Indicators should only appear when the current view actually overflows, and switching views should reset the scroll position appropriately.

Media

Additional context
The current usage dialog derives the indicators from data.models.length > 5, which does not describe trends or session content. The shared scroll state already exposes checkOverflow(), but it is not refreshed for every content transition.