Presets — named, switchable configurations for different keyboards

[engineer-myoa]

I've implemented presets for Rectangle and would like to check whether it's something you'd
consider before opening a PR, as CONTRIBUTING.md suggests. If it isn't a direction you want,
no problem at all — I'd rather hear that now than have you spend review time on it.

What I ran into

I switch between a full-size keyboard, a tenkeyless one, and the MacBook's built-in
keyboard. Most of my shortcuts sit on the numeric keypad, so on the other two they're
unreachable, and the bindings I'd want instead collide with the numpad set. Import/Export
can move a configuration between machines, but using it to switch several times a day means
two file dialogs each time, and there's no way to keep named sets around.

There's also one thing a config file can't express. MASShortcut distinguishes three states
per shortcut key: absent (the registered default applies), an empty dictionary (explicitly
unbound), and a real shortcut. Defaults.encoded() reads through
ShortcutCycle.shortcut(for:), which returns nil for the empty dictionary, so an explicitly
unbound shortcut isn't written to the file. With #1805 making import authoritative, the
omitted key is then removed on import and the built-in default applies again.

That matters here because a "tenkeyless" configuration is mostly defined by which bindings
should be off. #1805 did close the other half of what I originally hit — imports used to
leave omitted shortcuts alone — so this proposal is narrower than it would have been a week
ago.

What it does

A preset is a named snapshot of the shortcuts plus the settings that describe window
behavior. Settings about the app itself — launch at login, update checks, menu bar icon,
per-app exclusions, transient Todo state — stay global. The exclusion list is explicit and
tested against Defaults.array, so a future rename can't silently pull a key in.

Shortcuts are stored in all three states above, so switching restores unbound bindings as
unbound. Snapshots read persistentDomain(forName:) rather than dictionary(forKey:) to
tell an untouched shortcut from an assigned one.

The active preset simply mirrors the live settings, so there's no save button or dirty
state. Applying one writes settings first (so alternateDefaultShortcuts is current before
registerDefaults() re-runs), then shortcuts, then posts configImported and lets the
existing observers reconcile. Presets can be created from the built-in defaults or by
duplicating the current one, and renamed or deleted.

Footprint

One NSPopUpButton at the top right of the Shortcuts tab, added programmatically to the
existing stack view the way initializeAutoMaximizeCheckbox() does — no storyboard changes,
and no behavior change for anyone who ignores it. 11 new strings. 13 files, +949/−22, of
which 327 lines are two new files under Rectangle/Presets/ and 266 are tests.

If you'd rather it not be visible by default, I'm happy to put it behind a hidden preference
the way Todo mode is. Just say which you prefer and I'll adjust before opening the PR.

Presets ride along in Import/Export through a new optional Config.presets field rather
than another entry in defaults, which would have put the whole library into the file as
one escaped JSON string. It's the one place this steps outside the "every new key goes in
Defaults.array" convention; the convention's purpose — never silently dropping a key from
export — is still met. Both directions stay compatible and there are tests for it, and
Config keeps a memberwise initializer with presets defaulting to nil so existing call
sites are untouched.

Two small things came along the way: Defaults.load(fileUrl:)'s settings loop is extracted
to Defaults.apply(defaults:) so import and preset switching share a path, and
JSONDefault.load(from:) now clears its decoded value when a config sets the underlying
string to nil (previously footprint colors and snap areas couldn't be reset through import).
That second one is a different path from the init(key:defaultValue:) fix in #1811.

Testing

29 new tests, all passing; xcodebuild archive is green. For reference, f6279e2 runs 223
tests with 22 failing assertions across ActiveSideSplitRatiosCooperativeTests,
CooperativeCornerResizeTests and HalfSplitCornerCalculationTests, and my branch runs 252
with the same 22. I checked an unmodified checkout to be sure they aren't mine. Since
build.yml only archives, they wouldn't show up in CI — happy to open a separate issue if
that's useful.

[rxhanson]

Thanks for asking beforehand, and for looking to contribute! I do have some planned work from a previous PR that was closed that could touch some of the same areas (#1782), but from what I can tell it should be perfectly fine to roll in your pull request and continue. I'm up for reviewing it either way, and I suspect that I would be inclined to move forward with it. I think it's worth having in the UI, but my preference is to have it in the popover in the Extras button and the bottom of the General tab, not in the Shortcuts tab.

[engineer-myoa]

Thanks — that's great to hear, and thanks for the pointer to #1782.

I've moved the picker into the Extras popover, under a "Presets" header after the split-ratio rows, so the Shortcuts tab is untouched. PrefsViewController now only keeps a configImported observer so the shortcut recorders refresh when a preset is applied. No storyboard changes.

Opened it as #1819.

One small check on your wording: I read "the popover in the Extras button and the bottom of the General tab" as the one popover reached from the button at the bottom of General, so that's where it went. If you also wanted something directly on the General tab, say the word and I'll add it.

On #1782 — the only shared seam is Defaults.load(fileUrl:)'s settings loop, which I extracted into Defaults.apply(defaults:) so import and preset switching use one path. It's a pure extraction, so it should be easy to fold into a larger rework rather than fight with it.