Fix viewport lock — inner content scrolls, app frame fixed

[raggopaluni]

Description

App frame stays fixed like native app. Only inner content scrolls.

Fix:

• html, body → overflow: hidden, position: fixed
• Each screen → overflow-y: auto
• overscroll-behavior: none
• Prevent rubber band on iOS

Acceptance Criteria

• App frame never moves on scroll
• Content scrolls smoothly inside frame
• No bounce on iPhone
• No page shift on Android Chrome
• Bottom nav always fixed
• Tested on iPhone Safari and Android Chrome

Dependencies

Issue #3