fix(layout): cache pageWidth in localStorage to avoid render glitch

The page width from .remanso.json was only applied after an async
PouchDB + network fetch, so notes briefly rendered at the default
500px before snapping to the configured value. Persist pageWidth
alongside the existing font cache (key renamed to remanso:layout:*),
so it is read synchronously during setUserRepo and applied before
the first render. Also always reset --note-width with a default
fallback to prevent stale values leaking across repo navigation.

src/modules/user/hooks/useUserSettings.hook.ts

@@ -6,6 +6,7 @@ import { downloadFont } from "@/utils/downloadFont"
 
 const DEFAULT_FONT_POLICY = '"Libertinus Serif", serif'
 const DEFAULT_FONT_SIZE = "16px"
+const DEFAULT_NOTE_WIDTH = "500px"
 
 export const useUserSettings = () => {
   const store = useUserRepoStore()
@@ -24,10 +25,8 @@ export const useUserSettings = () => {
     )
     root.style.setProperty("--font-size", fontSize || DEFAULT_FONT_SIZE)
 
-    const pageWidth = store.userSettings?.pageWidth
-    if (pageWidth) {
-      root.style.setProperty("--note-width", pageWidth)
-      resetNoteWidthCache()
-    }
+    const pageWidth = store.userSettings?.pageWidth ?? DEFAULT_NOTE_WIDTH
+    root.style.setProperty("--note-width", pageWidth)
+    resetNoteWidthCache()
   })
 }