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/App.tsx

@@ -113,7 +113,7 @@ export function App() {
           )}
           <div style={{ flex: 1, minHeight: 0 }}>
             {active
-              ? <LayoutEngine workspaceId={active.id} layout={active.layout} running={running} states={states} surfaces={active.surfaces} focusedId={effectiveFocus} onFocus={setFocusedId} />
+              ? <LayoutEngine workspaceId={active.id} layout={active.layout} running={running} states={states} surfaces={active.surfaces} focusedId={effectiveFocus} onFocus={setFocusedId} zoomed={active.zoomed} />
               : <div style={{ color: COLORS.textMuted, padding: 24 }}>No workspace — create one to begin.</div>}
           </div>
         </div>