fix: make global search results keyboard navigable (#150)

* fix: add periodic health polling to prevent stale sidebar status dots

checkConnectionHealth was defined but never called after init().
Add a 30s setInterval useEffect so health status updates automatically
without requiring a page refresh.

Fixes #144

* fix: add aria-labels to Toolbar icon-only buttons for screen reader accessibility

View mode and theme dropdown triggers had aria-haspopup/aria-expanded
but no aria-label — screen readers announced them as generic 'button'.
Decorative SVG icons and color dots inside buttons were also read aloud
causing double-announcement.

Changes:
- Add dynamic aria-label to view mode dropdown trigger
- Add dynamic aria-label to theme dropdown trigger
- Add aria-hidden to decorative icons inside view/theme dropdowns
- Add aria-hidden to ChevronIcon in both dropdown triggers
- Add aria-hidden to color dot divs in theme dropdown items

Fixes #143

* fix: make global search results keyboard navigable

Search result divs had onClick but no role, tabIndex, or onKeyDown.
Keyboard-only users could not Tab to results or activate them.

- Add role=button to each result div
- Add tabIndex=0 to make results focusable via Tab
- Add onKeyDown handler for Enter and Space keys
- Add aria-label announcing table name and match count
- Add cursor:pointer for visual consistency

Fixes #142

Author: Nakshathra-2808