Spin the workspace emoji as the agent-working indicator

### Summary

Workspace icons can already be set to an emoji (Settings → Repository → Icon → Emoji). When an agent is working in that workspace, the sidebar currently shows a generic yellow ring spinner regardless of the icon you picked.

**Proposal:** when a workspace's icon is an emoji, use that emoji *as* its working indicator — rotate it while an agent runs, then settle back to the normal status dot. Workspaces without an emoji icon keep the existing ring spinner unchanged.

### Why

- Makes the "who is working" glance instant and personal — you recognize the workspace by the emoji you chose, not a row of identical rings.
- Reuses an existing, sanitized field (`repoIcon.emoji`); no new config, no new persistence.
- Purely additive and backwards compatible.

### Notes

- Rotation is a compositor-only CSS transform (no per-frame JS), phase-synced across mounts, mirroring the existing `AgentWorkingSpinner`.
- Disabled under `prefers-reduced-motion`.
- Cross-platform (renderer-only), SSH/remote-safe (reads the same status store), no provider-specific logic.

Happy to open a PR for this.

Start in the renderer's sidebar workspace status indicator and compare its working state with the existing AgentWorkingSpinner. Trace the existing repoIcon.emoji and status store usage, then verify that emoji workspaces animate during agent work, respect prefers-reduced-motion, and return to the normal status dot while workspaces without emojis retain the ring spinner.

Written by the indexing model from the issue text.