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/gui/window.hpp

@@ -49,6 +49,14 @@ private:
     void cancelLoad();
     void appendSourceRow(const std::string& path);
     void updateSourcesLabel();  // the collapsed expander shows the count
+
+    Gtk::Widget& buildCanvasSection();
+    // "" while "Custom" is picked, otherwise the format's key.
+    std::string canvasFormatKey() const;
+    void setCanvasFormat(const std::string& key, int dpi);  // dropdown and dpi
+    // A format owns the pixel sizes, so the spins follow it and are greyed
+    // out; "Custom" hands them back to the user untouched.
+    void syncCanvasWidgets();
     void removeSource(Gtk::ListBoxRow* row);
 
     Gtk::Widget& buildChainSection();
@@ -146,8 +154,14 @@ private:
     Gtk::Label* loadFileLabel_ = nullptr;
     Gtk::Expander sourcesExpander_;
     Gtk::ListBox sourceList_;
+    Gtk::DropDown formatDrop_;
     Gtk::SpinButton widthSpin_;
     Gtk::SpinButton heightSpin_;
+    Gtk::SpinButton dpiSpin_;
+    Gtk::Label* dpiLabel_ = nullptr;  // hidden along with the dpi spin
+    // Set while a format fills the spins, so their own handler does not queue
+    // a second render on top of the one the format change asks for.
+    bool suspendCanvasSync_ = false;
     Gtk::Grid paramGrid_;
     Gtk::Label paramHeading_;
     Gtk::Entry seedEntry_;