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>

readme.md

@@ -368,6 +368,33 @@ state of their own, so they can be read on their own.
 
 ---
 
+## The writing surface
+
+A real sheet, lying on a desk. It had been 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 rather than as paper.
+
+- **Warm off-white in daylight** (`#fbfaf7`). Plain white next to a white toolbar
+  is not a sheet, it is the absence of one.
+- **A raised surface at night** (`#1b2430`), lighter than the desk, with an edge
+  and a shadow — deliberately not a white rectangle. A page that glows brighter
+  than the room is what makes an app unusable in the evening; in a dark theme
+  "paper" means *the surface I write on*, not *white*.
+- **About 73 characters to the line**, measured rather than guessed: 620px of
+  sheet with a 56px margin. It ran to 84 before, 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.
+- **Centred**, with almost square corners. It used to sit 28px from the left and
+  stop short of the right, and carried a 14px radius that belongs to a dialog.
+
+Blocks *on* the sheet — quotes, code, table headers — are tinted from paper
+rather than from the app background, or they would read as sitting underneath it.
+
+The sheet keeps the drawing's page proportion, so a short note leaves an empty
+page below it. That is what a sheet of paper does, and it is the price of having
+one: the canvas needs a fixed page for a stroke to land in the same place on
+every screen.
+
 ## Interface language
 
 English throughout. It had drifted into two languages — the frame in English,