Offer standard web and print canvas formats

The canvas was two spin buttons, so "an Instagram story" or "A4 at 300 dpi"
was a number the user had to work out. src/core/canvas.cpp now holds one
table of standard sizes both front-ends read: a web format is a fixed pixel
size, a print format a sheet in millimetres that becomes pixels at a dpi.
Adding a format is a line in canvasFormats() — the dropdown, --format and
--help all walk the table.

The pixels stay the truth everywhere; renderChain never hears about formats.
A format is the presentation of a canvas, carried alongside the pixels in
Options and Project and empty for a custom size, and where one is named it
decides the pixels — so a project saved as A4 reopens as A4 rather than as
whatever it happened to measure.

In the interface a format fills the width and height in and greys them out
rather than hiding them: the size a sheet works out to is worth seeing. A
print format also shows a DPI row, and Custom hands the spins back keeping
the last size, which makes starting from a format and then tweaking it the
ordinary path.

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

CLAUDE.md

@@ -80,6 +80,17 @@ CLI builds it from repeated `-e`: `-e scanlines --strip-height 6 -e blur
 (`src/cli/options.cpp`) hands each flag to the stage whose effect declares it —
 two effects must not declare the same param key.
 
+### The canvas is a format or plain pixels
+
+`src/core/canvas.cpp` holds the one table of standard sizes both front-ends
+offer: web formats are a fixed pixel size, print formats a sheet in millimetres
+that becomes pixels at a dpi. Adding a format is a line in `canvasFormats()` —
+the dropdown, `--format` and `--help` all walk that table. The pixels stay the
+truth everywhere (`renderChain` never hears about formats); a format is the
+*presentation* of a canvas, carried alongside the pixels in `Options` and
+`Project` and empty for a custom size. Where a format is named it decides the
+pixels, so a project saved as A4 reopens as A4.
+
 ### Projects are plain JSON
 
 `src/core/project.cpp` reads and writes the whole setup — source paths, the