feat(config): add pageWidth setting to .remanso.json

Allows repo owners to configure note column width via `"pageWidth": "700px"` in .remanso.json. Applies the value to the --note-width CSS variable and invalidates the cached width so resize/overlay hooks pick it up.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/constants/note-width.ts

@@ -10,3 +10,7 @@ export const getNoteWidth = () => {
   }
   return cached
 }
+
+export const resetNoteWidthCache = () => {
+  cached = undefined
+}