Set up PWA manifest and service worker

[raggopaluni]

Description

Make NeighborPickd installable on any phone. Fullscreen, no browser bar, app icon.

Config:

• name: "NeighborPickd"
• theme_color: "#10b981"
• display: "standalone"
• Icons: 192x192 and 512x512

Behavior:

• First mobile visit → Add to Home Screen prompt
• After install → fullscreen, no Chrome bar
• Basic offline message if no connection

Acceptance Criteria

• Install prompt appears on mobile
• Opens fullscreen after install
• App icon on home screen
• Splash screen on launch
• Works on iPhone Safari
• Works on Android Chrome

Dependencies

Issue #3