Linear mode: missing error state, timeout hack, incomplete controls

## Recommendation

### 🔧 FINISH: Complete linear mode implementation

Three incomplete areas in the linear mode feature:

## What Was Found

### Missing Error State Indicator
- `src/renderer/extensions/linearMode/LinearControls.vue:133` — `//TODO: Error state indicator for failed queue?` — The `finally` block resets state without any error feedback to the user.

### Timeout Hack for Rerun
- `src/renderer/extensions/linearMode/LinearPreview.vue:90-92` — `//FIXME don't use timeouts here` — Uses `setTimeout(r, 500)` before widget callbacks. Comment notes "seeds fail to properly update even with timeouts."

### Incomplete 3D Preview Controls
- `src/renderer/extensions/linearMode/Preview3d.vue:22` — `//TODO: refactor to add control buttons` — Despite importing control components, the integration is incomplete.

## Context

Linear mode is a newer feature with several incomplete aspects that affect user experience and reliability.

---

**Category:** Half-Implemented | **Confidence:** MEDIUM | **Tools:** Semantic analysis
_Filed by audit-code skill_

Part of #11022

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11030-Linear-mode-missing-error-state-timeout-hack-incomplete-controls-33e6d73d36508111bfbfdadff5aa7201) by [Unito](https://www.unito.io)