[BUG] Dictation mic button disappears from composer once there's text — regression of the v1.46388.1 fix (macOS 1.49585.0)

v1.46388.1 (2026-09-04) shipped: "Fixed the dictation microphone button disappearing from the chat composer once a message had text, an attached file, or a quoted reply." That behavior is back on 1.49585.0.

### Steps to reproduce
1. Open a chat (or Cowork task) with an empty composer — mic icon and its chevron are present.
2. Type any text.
3. Mic icon and chevron disappear; only the send control remains.
4. Select all and delete the text — mic icon returns immediately.

### Expected
Mic stays available with text in the composer, so dictation can be appended to already-typed text.

### Notes
- Reproduces in both regular chats and Cowork tasks.
- Persists after a full quit and relaunch, so it isn't a stale renderer.
- The chevron beside the mic on this build opens an audio *input device* picker, not a dictation/voice-mode switch — so this isn't a mode-toggle state issue.

### Environment
- macOS, Apple Silicon (arm64)
- Claude desktop 1.49585.0 (41ad1d), built 2026-09-08T05:21:03Z
- Electron 44.2.0

### Impact
Breaks the type -> think -> dictate-more workflow entirely; only fallback is macOS system dictation.

Start by reproducing the disappearing mic icon and chevron on macOS 1.49585.0, including after typing, attaching a file, and quoting a reply. Use the v1.46388.1 fix mentioned in the issue as the first code reference if it is available, and consider the issue done when dictation controls remain visible in regular chats and Cowork tasks with existing text.

索引モデルが issue の本文から書いたものです。