feat(profile): Implement LocalStorage preferences for anonymous users

[arunderwood]

Summary

Create React hook for managing user preferences in browser LocalStorage, enabling anonymous users to personalize their experience.

Parent Epic

Part of #309 (User Profiles & Personalization)

Dependencies

• Depends on: Profile schema ADR issue

Deliverables

usePreferences Hook

• Create src/main/frontend/hooks/usePreferences.ts:

  interface UserPreferences {
    callsign?: string;
    gridSquare?: string;        // e.g., "FN31pr"
    preferredBands?: string[];  // e.g., ["20m", "40m", "2m"]
    preferredModes?: string[];  // e.g., ["SSB", "FT8", "CW"]
  }
  
  export function usePreferences(): {
    preferences: UserPreferences;
    setPreferences: (prefs: Partial<UserPreferences>) => void;
    updatePreference: <K extends keyof UserPreferences>(key: K, value: UserPreferences[K]) => void;
    clearPreferences: () => void;
    isLoaded: boolean;
  }

Validation

• Grid square validation (Maidenhead format: 2-8 characters, pattern: [A-R]{2}[0-9]{2}([A-X]{2}([0-9]{2})?)?)
• Callsign format validation (basic: 1-10 alphanumeric chars)
• Band/mode validation against known values

Persistence

• Store preferences in localStorage under key nextskip:preferences
• Persist across browser sessions
• Handle storage quota exceeded gracefully

Testing

• Unit tests for hook in src/test/frontend/hooks/usePreferences.test.ts
• Tests for validation functions
• Tests for localStorage interaction

LocalStorage Schema

{
  "callsign": "W1AW",
  "gridSquare": "FN31pr",
  "preferredBands": ["20m", "40m"],
  "preferredModes": ["SSB", "FT8"]
}

Acceptance Criteria

• Anonymous users can set/retrieve preferences from LocalStorage
• Preferences persist across page reloads
• Invalid grid squares are rejected with error message
• Hook provides loading state for hydration