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-proto/src/message.rs

@@ -104,6 +104,8 @@ pub enum Cmd {
         unread: Option<bool>,
         #[serde(default, skip_serializing_if = "Option::is_none")]
         order: Option<u32>,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        pinned: Option<bool>,
     },
     CreateGroup { name: String, color: String },
     SetGroup {