UI: "返回 WorkHub" button above the composer looks like a stray full-width strip and is not vertically centered

### What happened

With the WorkHub feature enabled, every ordinary (non-WorkHub) Session shows a "返回 WorkHub" / "Return to WorkHub" button rendered directly above the composer input. It appears as a full-width strip spanning the whole reading measure — not as a compact navigation control — and it is not vertically centered or aligned with the composer region, so it reads as stray UI between the transcript and the input.

Expected: a compact, properly aligned "返回 WorkHub" control that lives in the composer area the same way its sibling controls do (e.g. an inline chip aligned with the composer plate/edge, or a control in the composer's own toolbar row) and matches the app's button/chip language in light/dark and zh/zh-TW/en.

### How to reproduce

1. Open Settings → Preferences and enable the WorkHub feature (`workHub.enabled`).
2. From the WorkHub surface, open any ordinary Session (or open a Session while WorkHub is enabled).
3. Look at the composer area: a full-width bordered "返回 WorkHub" button sits between the transcript and the composer input, hugging the composer with asymmetric spacing (no top margin after the transcript's spacing-6 padding, only `8px` below to the composer plate).

### Environment

- Maka version or commit: `9cb5cc93c` (main)
- OS and version: Linux
- Surface: Desktop
- Node.js version, if running from source: n/a (desktop app)

### Logs, screenshots, or additional context

Code references (root cause for the visual):
- Rendered in `apps/desktop/src/renderer/app-shell.tsx` (~L2636): the button is a direct child of `ChatSurfaceLayout`'s `composer` slot, gated by `workHubEnabled && navSelection.section === 'sessions' && activeId`.
- Styled by `.workhub-return` in `apps/desktop/src/renderer/styles/workhub.css` (~L286): `display: block`, `width: min(var(--maka-reading-measure), calc(100% - var(--space-8)))`, `margin: 0 auto 8px`, border + left-aligned text. These override the standard Astryx `secondary` / `sm` Button look, so the control matches neither a normal button nor the compact chips used elsewhere around the composer (`.maka-composer-revision-notice`, `.maka-composer-no-model-hint`).
- Introduced in `feat(desktop): add conservative WorkHub session router (#3497)` and kept through `feat(workhub): restore chat identity and reuse standard composer (#5003)`.

No logs attached; defect is purely visual. A screenshot from a running session can be attached on request.