Bug: iPhone Safari zooms into composer and crops input on focus

[claude-do]

Summary

On iPhone Safari, focusing the chat composer can trigger browser zoom and leave part of the composer cropped off-screen once the keyboard opens.

Environment

• Device: iPhone
• Browser: Safari
• Area: chat composer / message input

Steps to reproduce

1. Open the app on an iPhone in Safari.
2. Navigate to any chat thread.
3. Tap into the chat composer.
4. Wait for the on-screen keyboard to appear.

Actual behavior

Safari zooms into the page when the composer receives focus, and part of the composer can be cropped or pushed out of view.

Expected behavior

Focusing the composer should keep the input fully visible without triggering browser zoom. Users should still be able to zoom the page manually when they are not focused inside the composer.

Likely cause

The composer renders at 14px on mobile. On iPhone Safari, editable controls below 16px commonly trigger focus zoom.

Proposed fix

Use 16px composer text on mobile and keep the existing smaller composer sizing from sm and up.

Before/after phone recording checklist

Before (current broken behavior)

1. Start screen recording on iPhone.
2. Open the app in Safari at normal page scale.
3. Show the full thread view and composer.
4. Tap into the composer.
5. Capture the moment Safari zooms in.
6. Show that part of the composer is cropped or harder to use once the keyboard is open.

After (fixed behavior)

1. Start screen recording on the build with the fix.
2. Open the same thread in Safari.
3. Show the full thread view and composer before focus.
4. Tap into the composer.
5. Capture that the keyboard opens without Safari zooming the page.
6. Show that the composer remains fully visible and usable.
7. Optionally pinch-zoom elsewhere on the page to demonstrate that manual zoom is still available outside the focused input.

Notes

This is worth documenting separately from general small-screen layout issues because it is specifically about iPhone Safari focus zoom behavior and accessibility-sensitive zoom expectations.

[claude-do]

Proposed PR title:

Fix mobile composer focus zoom on iPhone Safari

Proposed PR message:

Summary

• use 16px composer text on mobile to avoid iPhone Safari focus zoom
• keep the existing smaller composer sizing from sm and up
• update the placeholder sizing to match the editor text size

Why this change

The composer was rendering at 14px on mobile, which can trigger Safari's automatic focus zoom for editable fields on iPhone. That zoom can make the composer feel cropped or partially off-screen once the keyboard opens.

Testing

• open the chat view on iPhone Safari
• tap into the composer and confirm the keyboard opens without page zoom
• confirm the composer remains fully visible while focused
• confirm desktop sizing is unchanged
• /home/claude/.bun/bin/bun fmt
• /home/claude/.bun/bin/bun lint
• env PATH="/home/claude/.bun/bin:$PATH" /home/claude/.bun/bin/bun typecheck