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>

README.md

@@ -34,6 +34,22 @@ build/effector -o out.png -s 1920x1080 \
     --strip-height 8 --jitter 4 --seed 42 photo1.jpg photo2.png
 ```
 
+The canvas is either a size in pixels (`-s 1920x1080`, the Custom entry in the
+interface) or one of the standard formats, which `--help` lists in full:
+
+```bash
+build/effector -o post.png --format instagram-post -e tiles
+build/effector -o sheet.png --format a4 --dpi 300 -e scanlines
+```
+
+A web format — the Instagram sizes, Full HD, 4K UHD — is a fixed pixel size. A
+print format — A5, A4, A3, US Letter — is a sheet of paper, and becomes pixels
+at `--dpi` (36–1200, 300 by default), so A4 at 300 dpi is 2480×3508. Picking a
+format in the interface fills the width and height in and greys them out;
+Custom hands them back, keeping whatever the format last worked out to. A
+format is saved with the project, so an A4 project reopens as A4 rather than as
+the pixels it happened to be.
+
 Any option can also be passed when launching the interface; it becomes the
 starting state (images, canvas size, effect settings, seed).