bug(desktop): composer caret appears as a tiny mark after switching conversations from WorkHub

## What happened

After navigating through WorkHub and switching conversations in the sidebar, the insertion caret at the start of the empty conversation composer appears as a tiny dot or short segment above the placeholder, instead of a full-height caret.

The placeholder is “描述任务，@ 引用文件，/ 选择技能…”. The problem concerns the text insertion caret, not the mouse pointer or an agent activity indicator.

Expected: when the empty composer is focused, the insertion caret should be fully visible alongside the placeholder.

## How to reproduce

The reporter observed the following sequence:

1. Open Desktop with several existing conversations and enter WorkHub.
2. Click a task card in the horizontal row at the top of WorkHub.
3. Click a conversation in the left sidebar, then switch between sidebar conversations twice in succession. The repeated action is the sidebar switching, not the entire WorkHub sequence.
4. Focus the empty conversation composer and inspect the caret before the placeholder.

The reporter reproduced the visual problem on macOS and initially observed it on Windows. The exact timing requirement and reproduction rate have not yet been established.
## Environment

- Maka Desktop: 0.2.0, source baseline `d2e6c1f273b7daa23ff75b84c8a30dca3f316713`.
- macOS: 26.5.2, arm64.
- Electron dependency: 43.4.1.
- UI dependency: `@astryxdesign/core` 0.5.2.
- Also reported on Windows; Windows version has not been collected.
- The local checkout contains separate provider-request diagnostic changes. No composer or WorkHub code was changed for this investigation; this is not yet a clean-checkout reproduction.

## Logs, screenshots, or additional context

The cause is not established: clipping, caret geometry, and selection restoration are investigation candidates, not confirmed diagnoses. Code inspection found draft-selection restoration in `packages/ui/src/composer.tsx`; the existing focus/selection unit tests do not establish the visual caret height in Electron.

Related but not known duplicates: #4827 / #4836 concern caret position after changing thinking level; #4602 addresses restored drafts taking focus. None establishes a fix for this reported visual symptom.

Draft prepared with OpenAI Codex from the reporter's reproduction steps and screenshots, local code inspection, and issue/PR searches. Independent automated reproduction has not been established.

### Screenshot

The insertion caret immediately before “描述任务” is visible only as a short mark above the placeholder, rather than a full-height vertical line.