Add keyboard shortcuts for common actions

[MeisnerDan]

Implement keyboard shortcuts to accelerate workflow. Users should be able to create a new task with Cmd+N (Mac) / Ctrl+N (Linux/Windows), focus the task search with Cmd+K / Ctrl+K, and save/submit forms with Cmd+Enter / Ctrl+Enter. This will improve power user experience and make the app more competitive with other productivity tools.

Acceptance Criteria

• Cmd/Ctrl+N opens the create task modal
• Cmd/Ctrl+K focuses the global search/filter input
• Cmd/Ctrl+Enter submits active forms (task creation, task editing)
• Keyboard shortcuts work consistently across all pages
• Shortcuts are documented in a help overlay (accessible via ? key)
• No shortcuts conflict with browser defaults (test in Chrome, Firefox, Safari)
• Unit tests for shortcut handlers in useKeyboardShortcuts hook