Previously, disabling Clipy's automatic checks skipped startUpdater(),
which also prevented manual update checks. Always start the updater after
clearing the legacy feed URL, leaving scheduling and permission prompts
to Sparkle.

Automatic checks and check intervals were also stored in separate Clipy
preferences. Answers to Sparkle's permission prompt did not update Clipy's
preference, and interval changes were only applied when relaunching the
app. Remove the duplicate keys and expose Sparkle's persisted values
through setters and KVO publishers so consumers can observe and update
the same settings.

Expose canCheckForUpdates and a typed, optional lastUpdateCheckDate to
reflect updater readiness and the absence of previous checks.

Delete the old preference controllers, XIBs, localized resources, and their Xcode project references after switching to SwiftUI settings panes.

Remove unused preference images and colors, the designable button, obsolete localization keys, and exclusion list mutation helpers used only by the old UI.

Keep numeric input in local draft state while retaining the standard TextField formatter. Apply changes on submission, focus loss, or pane changes, and preserve immediate stepper updates.

End editing when the settings window closes so pending values are saved. This prevents intermediate history limits from reaching the periodic history cleanup while the user is still typing.

Preserve each pane’s ideal height while allowing content to scroll when it exceeds the available screen height. Recalculate the height limit when the window changes screens and reposition windows that would extend beyond the visible screen.