feat(app): panel zoom — full-grid render + header toggle

Wire Cmd::SetZoom through Tauri bridge (set_zoom command), add zoomed
field to WorkspaceView, short-circuit LayoutEngine to render only the
zoomed panel full-grid, and toggle Maximize2/Minimize2 in panel header.

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

app/src-tauri/src/bridge.rs

@@ -299,6 +299,17 @@ pub async fn focus(state: BridgeState<'_>, surface_id: String) -> Result<Value,
     data_of(state.request(Cmd::Focus { surface_id: SurfaceId(surface_id) }).await.map_err(|e| e.to_string())?)
 }
 
+// ---- SP4 zoom command ----
+
+#[tauri::command]
+pub async fn set_zoom(state: BridgeState<'_>, workspace_id: String, surface_id: Option<String>) -> Result<Value, String> {
+    let cmd = Cmd::SetZoom {
+        workspace_id: spacesh_proto::WorkspaceId(workspace_id),
+        surface_id: surface_id.map(spacesh_proto::SurfaceId),
+    };
+    data_of(state.request(cmd).await.map_err(|e| e.to_string())?)
+}
+
 // ---- M3 event log commands ----
 
 #[tauri::command]