feat: global search (Cmd+K) and filter panel for work items

[DorianZheng]

Summary

Adds a local work item search and filtering experience for the Dune desktop app.

Requirements addressed

1. Global search bar (Cmd+K): Cmd+K / Ctrl+K opens a full-screen search palette for work items.
2. Filter panel: Workflow board includes filters for status, assigned agent, created/updated date range, and reviewer signal.
3. Search results: Results show item title, status, assignee, and a matching snippet.
4. Combinable filters: Search and board filtering use the shared filter state, so filters combine across status, agent, date, and reviewer state.
5. Local search: SearchIndex builds an in-memory index from loaded items and work products; no backend dependency.
6. Keyboard navigation: Palette supports ArrowUp/ArrowDown selection, Enter to open an item, and Escape/dialog close behavior.

Validation

• npx tsc --noEmit
• npx vitest run tests/unit/src/renderer/utils/searchIndex.test.ts