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/spacesh-cli/src/mapping.rs

@@ -61,6 +61,7 @@ pub fn to_cmd(sub: Sub) -> Cmd {
             group_id: group.map(|g| if g.is_empty() { None } else { Some(GroupId(g)) }),
             unread,
             order,
+            pinned: None,
         },
         Sub::Shutdown => Cmd::Shutdown,
         Sub::Completions { .. } => unreachable!("completions handled before dispatch"),