fix(a11y): remediate SortSite round-2 accessibility findings

[MasonStilwell]

Remediates five accessibility findings from the SortSite review. Verified locally (lint clean, full unit suite 2568 passing, production build succeeds) and deployed to the dev site for manual keyboard/screen-reader verification before this promotion to live.

Findings addressed

1. User menu focus (Item 1) — Opening the sidebar user/settings menu now moves focus to the first item, both triggers advertise aria-haspopup/aria-expanded/aria-controls (gated per-trigger), and focus returns to the opener on dismiss.
2. Toolbar keyboard reachability (Item 2) — Refresh, the status/action icon buttons, and the "Search All Fields" input now join the data-grid toolbar's ARIA roving-tabindex group (W3C toolbar pattern): Tab enters the toolbar, arrow keys move between controls; the search input is reachable when empty.
3. Empty "More" menu (Item 3) — The kebab "More actions" button is removed from the DOM entirely (invisible to sighted and screen-reader users) when it has no menu items, e.g. on Stem Codes.
4. GitHub Feedback dialog announcement (Item 4) — The dialog exposes a stable accessible name and description (aria-labelledby/aria-describedby) across all states, so screen readers announce it immediately on open, matching the Upload dialog.
5. Table accessible names (Item 5) — Every data grid now sets a meaningful aria-label (e.g. "Stem Codes") via a shared getGridTypeLabel map, instead of the screen reader concatenating header/cell text.

Notes

• Toolbar focus uses the ARIA roving-tabindex pattern (one Tab stop + arrow-key navigation). This is the standards-correct model; automated scanners that don't simulate arrow keys may still flag the tabindex=-1 controls.
• Keyboard and screen-reader behavior should be confirmed on the dev deploy before merging to live.