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>

src/cli/options.hpp

@@ -5,6 +5,7 @@
 #include <string>
 #include <vector>
 
+#include "core/canvas.hpp"
 #include "core/chain.hpp"
 #include "core/params.hpp"
 
@@ -16,6 +17,8 @@ struct Options {
     std::string output;              // empty -> launch the interface
     int width = 1920;
     int height = 1080;
+    std::string format;      // a named canvas format, empty for plain pixels
+    int dpi = kDefaultDpi;   // only a print format is rendered at a resolution
     std::optional<std::uint64_t> seed;
     ParamValues params;              // only the keys the user actually passed
     bool listEffects = false;