Make the writing surface a sheet of paper

It was a page-shaped box that was not page-coloured: the same fill as everything
around it with a hairline border, so it read as an empty panel. Now it is warm
off-white in daylight and a raised surface at night — deliberately not a white
rectangle, since a page that glows brighter than the room is what makes an app
unusable in the evening.

The line ran to 84 characters, past the width at which the eye reliably finds the
start of the next line, and that was the single biggest reason the surface read
badly. Measured, not guessed: 620px of sheet with a 56px margin gives about 73.
It is centred now — it used to sit 28px from the left and stop short of the
right — with almost square corners instead of a dialog's 14px radius.

Blocks on the sheet are tinted from paper rather than from the app background,
or they read as sitting underneath it.

Five German strings survived the earlier pass, "Alle Notizen exportieren" among
them: that sweep filtered on umlauts, so sentences without one went straight
through. Swept again by word.

218 unit tests, 244 browser checks across eleven suites.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/ui/AccountMenu.jsx

@@ -386,7 +386,7 @@ export function AccountMenu({ user, onClose, onLogout, onUserUpdate, onOpenAdmin
               marginBottom: 8,
             }}
           >
-            {I.export} Alle Notizen exportieren
+            {I.export} Export all notes
           </button>
           {user?.isAdmin && (
             <button