feat: add command palette for navigation

[shogun444]

Description

Closes #726

Adds a keyboard-accessible command palette for application navigation.

What changed

• Added a new CommandPalette component mounted from src/app/layout.tsx
• Added global Cmd+K / Ctrl+K keyboard shortcut to open the palette
• Added searchable navigation entries for:
  • Home
  • Contracts
  • Milestones
  • Reputation
• Implemented keyboard navigation with:
  • Arrow Up / Arrow Down
  • Enter to navigate
  • Escape to close
• Reused the existing useDialogFocusTrap hook for focus management and restoration
• Added reduced-motion support using the existing useMediaQuery hook
• Prevented the shortcut from opening while typing inside editable elements (input, textarea, select, and contenteditable)
• Ensured duplicate navigation results are not displayed
• Added comprehensive tests covering keyboard interaction, filtering, navigation, accessibility behavior, and focus management

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Type of Change

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional change, internal code improvement)
• Documentation update

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Pre-flight Checklist

• npm run lint passes with no errors
• npm test passes with no failures
• npm run build completes successfully

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Testing & Coverage

• Module test coverage for impacted files meets or exceeds the 95% minimum threshold
• If this PR adds or modifies UI components, accessibility tests were written using the shared utilities in src/test-utils/a11y.tsx

What was tested?

Added automated tests covering:

• Opening the palette with Cmd+K and Ctrl+K
• Preventing the shortcut while typing in editable fields
• Navigation filtering by label and keywords
• Keyboard navigation using Arrow keys
• Navigation via Enter
• Mouse selection
• Closing with Escape
• Focus restoration after closing
• Duplicate result prevention
• Empty-state rendering
• Reduced-motion behavior

A total of 18 new tests were added.

Project verification:

• npm run lint
• npm test (1278 tests across 74 suites passing)
• npm run build

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Accessibility & Security Notes

Accessibility

• Keyboard accessible via Cmd+K / Ctrl+K
• Proper combobox, listbox, and option ARIA roles
• aria-activedescendant used for active option tracking
• Focus is trapped while the palette is open and restored on close
• Reduced-motion preferences are respected

Security

N/A — this change only adds client-side navigation functionality and does not modify authentication, wallet logic, authorization, or API behavior.

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