[BUG]: On-screen keyboard covers composer input on mobile web (portrait)

Description

On the opencode web UI on mobile (portrait), when the on-screen keyboard opens to type a prompt, the composer/message input box is covered by the keyboard. Only the top sliver of the input (and the text being typed) remains visible, so it is impossible to see or edit what you are typing.

With the keyboard dismissed, the input box renders correctly — the problem only appears while the keyboard is shown.

Steps to Reproduce

1. Open the opencode web UI on a mobile device (Android) in portrait orientation.
2. Tap the composer input to focus it and bring up the on-screen keyboard.
3. Type a message.
4. The on-screen keyboard covers the composer input box; the input does not scroll/stay above the keyboard.

Expected Behavior

When the on-screen keyboard is open, the composer should remain fully visible above the keyboard (the viewport should resize to the visual viewport, or the layout should be pushed up).

Actual Behavior

The keyboard overlays the composer input. The input box is only partially visible (its top edge peeks out above the keyboard), so the user cannot see the full input or the text they are typing.

Screenshots

• Keyboard open: composer input is covered by the keyboard (only the top of the input box is visible).
• Keyboard closed: composer input renders normally.

Environment

• opencode web UI (v2 layout)
• Android Chrome, portrait mode
• Landscape/desktop are reportedly unaffected

Additional Context

This looks related to, but is distinct from, #37545 (temperature/effort dropdown overlaying the composer). That issue is about the dropdown; this one is about the on-screen keyboard covering the input itself. Possibly the same underlying mobile viewport/interactive-widget handling (resizes-visual vs resizes-content).