[chinzu00]

Summary

The canvas's shortcuts were a match in the core: which press did what was fixed, and the list in
Help could only show it. They are a table now, and the list binds a row's keys when they are
clicked. Press the new key to bind it, Backspace to clear it, Esc to give up; Reset restores the
app's own. The bindings are kept between runs with the other preferences.

This is the desktop half of #81 (customizable shortcuts); the core side is exported for Android in
KeyBindings, but no Android UI for it is in this PR.

What changed

Core — crates/core/src/keys.rs

• KeyAction names what a press can do (Undo, Paint, Layers, …). Brush size and arrow nudging stay
  out: they take a step from the modifiers rather than being one command.
• KeyBinding/KeyBindings hold the presses each action answers to. They start from the shortcuts'
  table itself, so the list and the keys cannot drift apart.
  bind() takes a press from whatever had it, unbind() clears an action, reset() restores the
  start, read_line()/kept_lines() read and write them in the app's settings file.
• key_shortcut_bound() reads the presses as they are now, keeping the reading the canvas has
  always had: the exact press, then with Ctrl the same press without Shift, then the key alone, then
  the presses that take a step. Ctrl+Alt (AltGr) still types rather than binding, and Ctrl+Space
  still does nothing.
• key_shortcut() and key_shortcut_list() keep their signatures for the Android bindings and mean
  "with the presses the app starts with"; key_shortcut_with()/key_shortcut_list_with() take
  bindings instead. ShortcutRow gained action, so a caller knows which row can be bound.

Desktop — desktop/src/

• prefs.rs: Prefs::keys, read from and written to interface as key_undo ctrl+z,ctrl+shift+z
  lines (an empty value is unbound).
• ui.rs: the canvas's keys go through prefs.keys; the shortcuts panel takes the keys while it is
  open, so a key being chosen does not also paint or undo.
• shortcuts.rs: a shortcut's row is one target — its keys and its name light together while the
  pointer is over either, and a click anywhere on the row asks for a key ("Press a key…"). The list
  shows what the bindings have now ("Unbound" when an action has none), and rises into place: a
  150 ms fade and a 16-point lift, easing out. Reset and a hint line sit by Done.

Testing

• cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings pass.
• cargo test --workspace passes: 566 tests. On this machine the two plugin_filter tests that use
  a real GPU fail as they always have here (a_filter_opens_redraws_as_it_slides_and_cancel_puts_the_layer_back,
  reset_preview_and_undo_keep_it_open_at_its_start); CI's runner has no GPU and skips them.
• New tests in keys.rs: the presses bound do what they did before they could be bound; binding a
  key adds it without changing the rest; a press bound again is taken from what had it; an action
  unbound has no keys and does nothing; the bindings keep and come back as they were (settings-file
  round trip); a press is written and read back.
• Tried by hand on Windows with the release build: opened Help › Keyboard shortcuts, clicked Erase's
  keys, pressed F (row read E F, interface gained key_erase e,f), pressed Done and F
  selected the eraser; a binding saved before a restart was shown again after it.
• Held Ctrl with no other key: the list came up after 0.9 s and went on release; turned
  Shortcut list on Ctrl held off in Prefs (shortcuts_on_ctrl false in interface) and held
  Ctrl again: nothing came up.
• The rise: a capture 20 ms in shows the panel half-faded and 16 points low; 0.3 s later it is
  solid and in place. Hovering a row's name lights its keys too, and clicking the name asks for
  the key.

Ctrl (⌘) held alone shows it

That way of showing the list never worked: Shortcuts::show counted "another key is down" as
!keys_down.is_empty(), and egui puts the modifier keys themselves in that set (Key::ControlLeft
and its kind), so the hold was spoiled on its first frame. What spoils the hold now is a key that
is not a modifier (Shift, Ctrl, Alt or Super, either side); a click or a scroll still does. The
list comes up after 0.9 s and rises in.

A new Prefs switch, Shortcut list on Ctrl held (shortcuts_on_ctrl, on by default), turns it
off for anyone who wants the list from Help alone. It is kept with the other preferences, so a new
line joins the file: shortcuts_on_ctrl true or false.

Part of #81 — the desktop side; the Android UI for the same bindings is still to come.