fix(mobile): remove stray composer text

[rynfar]

Problem

ThreadComposer emitted an explicit whitespace text child between the attachment modal and image viewer. React Native requires raw strings to be inside <Text>, so opening an ordinary thread could produce Text strings must be rendered within a <Text> component. even though the composer otherwise rendered correctly.

Fix

Remove the stray JSX whitespace child. The null-submission crash originally included in this PR was independently merged in #38, so this branch was rebased and reduced to its one remaining unique fix instead of duplicating that logic.

Verification

• targeted lint for ThreadComposer.tsx
• vp run -F @t3tools/mobile typecheck
• git diff --check
• integrated iOS Simulator reproduction from the original review rendered the seeded ordinary thread without the raw-text-child warning

Model: GPT-5.6; Harness: Prime Agent.

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for cc34dc5.

This comment will update automatically after the next completed run.