feat: expand command palette actions and improve navigation UX

[clFaster]

Summary

• implement issue Expand command palette actions (rename, duplicate, delete, import/export boards, toggle timestamps) #18 command palette expansion with board-scoped actions (open, rename, duplicate, delete)
• change open board to a VS Code-style two-step command flow with a filterable board picker
• add import/export boards and timestamp toggle to command palette actions
• improve command palette UX with VS Code-like scrollbar behavior and distinct hover vs keyboard selection states
• keep keyboard-selected item visible while navigating and prevent passive mouse hover from stealing keyboard selection

Testing

• pnpm format
• pnpm lint
• pnpm test:system

Closes #18