fix: stabilize responsive navigation and database toolbar

[JRebertt]

Summary

• measure CSS-hidden overflow tabs at their natural width to prevent the More menu from oscillating
• keep mobile topbar actions accessible through horizontal scrolling
• reset the More trigger appearance so it follows light and dark themes
• keep the Database Explorer title and actions aligned in one scrollable toolbar on small screens

Root cause

The overflow hook only detected items hidden with an inline display: none. PageTopbar hides overflowed tabs through a CSS class, so their measured width became zero and repeated resize observations alternated between expanded and collapsed states. Mobile toolbars also allowed fixed-width action groups to compress or extend beyond the viewport without a scroll container.

Validation

• npm test (158 passing)
• overflow regression test repeated 5 times
• npm run build
• npm run lint (0 errors)
• browser checks at 320, 375, 390, 430, 640, 768, 900, 1000, 1100, 1200, 1300, and 1400 px
• light and dark theme checks for the More trigger and mobile scrollbars
• verified the first and last mobile toolbar controls remain reachable without overlap