fix(bindx-ui): preserve keyboard navigation in cell tooltips

[matej21]

Closes #119.

Summary

Fix the keyboard-navigation regression introduced by the portaled cell filter panel in 0.1.52.

• Focus reveals the panel without moving focus away from the cell label or its interactive children.
• ArrowDown on the label enters the panel explicitly. Labels expose the shortcut through aria-keyshortcuts.
• Escape restores focus to the originating cell without reopening the panel, including when the panel was opened by hover.
• Tab and Shift+Tab at the panel edges bypass Radix's focus loop and let native navigation continue from the originating cell.
• Preserve hover grace periods and coordinated pointer/focus behavior.

Use tabbable to identify eligible focus targets instead of maintaining a custom focusability selector. Add a link to the playground's relation cell to exercise real navigation.

Verification

• Full unit/integration suite: 2107 passed, 0 failed.
• Chromium browser tests for keyboard navigation and the existing datagrid: 21 passed, 0 failed.
• Browser coverage includes native Tab/Shift+Tab, following the cell link with Enter, explicit ArrowDown entry, Escape restoration, reopening, and hover-opened panel dismissal.
• bun run typecheck passed.
• bun run lint --ignore-pattern '.claude/**' passed with 0 errors and 16 existing warnings. The ignored directory contains unrelated nested worktrees.