feat(spaceshd): pinned workspace field

Add a `pinned` bool to the workspace model, threaded through proto
(Workspace + WorkspaceView + SetWorkspaceMeta), the registry, the
set_workspace_meta handler, persistence, the CLI mapping, and the Tauri
bridge. serde(default) keeps existing state.json compatible (pinned=false).
Backs the sidebar Favorites section.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spaceshd/src/state_store.rs

@@ -92,6 +92,7 @@ mod tests {
                 group_id: None,
                 order: 0,
                 unread: false,
+                pinned: false,
                 layout: None,
                 zoomed: None,
                 surfaces: std::collections::HashMap::new(),