Implement fit-mode rendering contract with container sizing and pane re-fit

src/main.tsx

@@ -1,5 +1,6 @@
 import { createRoot } from 'react-dom/client';
 import { App } from './app/App';
+import { initPreviewFitMode, wirePreviewFitMode } from './app/orchestration/preferences';
 import { initApp } from './app/orchestration/startup';
 import { initTheme, wireTheme } from './app/orchestration/theme';
 import '../styles/base.css';
@@ -10,6 +11,11 @@ import '../styles/base.css';
 initTheme();
 wireTheme();
 
+// Hydrate + persist the preview fit mode the same way (pulled ahead of the M5
+// Settings modal); hydrating before render keeps the store authoritative.
+initPreviewFitMode();
+wirePreviewFitMode();
+
 // Load the library from IndexedDB (seeding a sample on first run) and wire
 // persistence. Fire-and-forget: the UI renders immediately and fills in when
 // hydration resolves.