Make displays configurable, with a revert countdown

Resolution, refresh rate, scale, and rotation, applied through
hl.monitor{} and stored per output.

This is the only setting in Panama where a wrong value can leave the
user unable to SEE the screen well enough to undo it: a mode the panel
cannot show, or a scale that makes everything unreadable, is not
recoverable through the UI that caused it. So a change is never applied
irreversibly. It is applied, then reverted automatically after fifteen
seconds unless confirmed, and confirming is what writes it to the
settings store -- letting the countdown run leaves nothing behind.

The contract tests that property specifically: it applies a scale, waits
out the countdown, and asserts the display came back and that nothing
was stored. A regression there is not a broken feature, it is a user
staring at a blank monitor.

Modes are grouped by resolution with refresh rates beside them. The
panel reports 35, many differing only in refresh-rate rounding -- 60.00
and 59.94 -- which as a flat list of buttons is noise rather than
choice; equal rounded pairs collapse, leaving 21.

Only mode, scale, and transform are configurable. Colour management and
bit depth stay in monitors.lua because they carry a documented screencopy
tradeoff that a settings page cannot explain at the moment you would be
changing it.

Also replaces the display policy rows with the schema-bound ones, so the
page no longer restates labels that PreferenceSchema already holds.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/modules/settings/qmldir

@@ -35,3 +35,5 @@ ApplicationsPage 1.0 ApplicationsPage.qml
 DockPinsEditor 1.0 DockPinsEditor.qml
 DockAppPicker 1.0 DockAppPicker.qml
 ShortcutCapture 1.0 ShortcutCapture.qml
+ChoiceGrid 1.0 ChoiceGrid.qml
+DisplayModePicker 1.0 DisplayModePicker.qml