feat(profile): Create preferences UI

[arunderwood]

Summary

Create the user interface for viewing and editing preferences.

Parent Epic

Part of #309 (User Profiles & Personalization)

Dependencies

• Depends on: User profile persistence issue

Deliverables

Settings Access

• Add settings/gear icon button to dashboard header
• Open preferences modal on click

Preferences Modal

• Create src/main/frontend/components/PreferencesModal.tsx
• Form fields for:
  • Callsign input (optional, with validation feedback)
  • Grid square input (with Maidenhead validation feedback)
  • Band selection (multi-select checkboxes: 160m, 80m, 40m, 30m, 20m, 17m, 15m, 12m, 10m, 6m, 2m, 70cm)
  • Mode selection (multi-select checkboxes: SSB, CW, FT8, FT4, RTTY, AM, FM, Digital)
• "Login to sync across devices" prompt for anonymous users
• Save/Cancel buttons
• Loading states during save
• Success/error feedback

Styling

• Create src/main/frontend/components/PreferencesModal.css
• Styling consistent with dashboard theme
• Responsive design (works on mobile)
• Accessible (keyboard navigation, ARIA labels)

Integration

• Update DashboardView.tsx with settings button in header
• Wire up to usePreferences hook

UI Mockup

┌─────────────────────────────────────────────────┐
│ ⚙️ Preferences                              [X] │
├─────────────────────────────────────────────────┤
│                                                 │
│ Callsign (optional)                             │
│ ┌─────────────────────────────────────────────┐ │
│ │ W1AW                                        │ │
│ └─────────────────────────────────────────────┘ │
│                                                 │
│ Grid Square                                     │
│ ┌─────────────────────────────────────────────┐ │
│ │ FN31pr                              ✓ Valid │ │
│ └─────────────────────────────────────────────┘ │
│                                                 │
│ Preferred Bands                                 │
│ ☑ 20m  ☑ 40m  ☐ 80m  ☐ 15m  ☑ 2m  ...        │
│                                                 │
│ Preferred Modes                                 │
│ ☑ SSB  ☑ FT8  ☐ CW  ☐ FM  ...                 │
│                                                 │
│ ┌─────────────────────────────────────────────┐ │
│ │ 💡 Login with GitHub to sync across devices │ │
│ └─────────────────────────────────────────────┘ │
│                                                 │
│                    [Cancel]  [Save Preferences] │
└─────────────────────────────────────────────────┘

Acceptance Criteria

• Users can view and edit preferences through the UI
• Validation feedback shown for invalid inputs
• Changes persisted to LocalStorage (and DB if logged in)
• Anonymous users see "login to sync" prompt
• Modal accessible via keyboard (Escape to close, Tab navigation)