fix(board): wire the task search keyboard shortcut

[afurm]

The board header shows a ⌘K hint next to task search, but the app does not currently focus the search input when the shortcut is pressed.

Why this is useful:

• New users expect visible keyboard hints to work.
• It is a small UI behavior change with a clear before/after.
• It gives first-time contributors a focused React test target.

Suggested scope:

• In src/client/App.tsx, add a keyboard handler for Meta+K on macOS and Ctrl+K elsewhere while the board view is active.
• Focus the visible task search input and prevent the browser default for that shortcut.
• Make sure the mobile/tablet search input path still behaves correctly.
• Keep normal typing inside inputs/textareas unaffected.

Acceptance criteria:

• Pressing ⌘K or Ctrl+K from the board focuses task search.
• The shortcut does not steal focus while typing in an input, textarea, or content-editable element.
• Add or update a focused Vitest/jsdom test for the shortcut behavior.
• npm run typecheck and npm test pass.