feat: the preferences are a UI file, applying as they change

src/ui/preferences.ui is an AdwPreferencesDialog whose rows are libadwaita's own -- AdwEntryRow,
AdwSpinRow, AdwSwitchRow, AdwComboRow, AdwButtonRow -- instead of a label and a bare GtkEntry in a
GtkBox. There is no Apply button: a row applies as it changes, which is what those widgets are
designed around, and [prefs-probe] reads the settings file back after every change.

The folder chooser is cancellable now, so a picker still open when the dialog closes cannot write
into a row that is gone; the cancellable is also how the probe takes the cancel path without a
mouse.

Two traps cost a run each and are recorded in docs/claude/invariants-gtk-and-chrome.md: an
AdwDialog is not a GtkWindow (it cannot parent a GtkFileDialog, and in floating mode it adds a
native surface to gtk_window_get_toplevels() that is neither a widget nor a window), and a .ui
that is not listed in the GResource is a Gtk-ERROR at runtime rather than a build failure -- which
is exactly what the probe opening every window is for.

README.md

@@ -609,8 +609,12 @@ all is the trigger pane's own switch, and there is deliberately no second one he
 the one place every source gets a folder inside, `<cache>/udpscope/events` by default. The cache
 directory and relative time are *start-up* settings -- the buffers are planned and the cache directory
 is opened before any source is known -- and the dialog says so; the theme applies as soon as it is
-chosen. A key that is absent means its default, so the file only mentions what changed, and a missing
-file is a first run rather than an error. The directory field shows the directory in use: the default
+chosen. A change
+applies as it is made — there is no Apply button, and a **Reset to defaults** row at the end of the
+dialog puts everything back — and the rows are the ones every GNOME application shows: a title, a
+subtitle under it where the setting needs explaining, and the control on the right. A key that is
+absent means its default, so the file only mentions what changed, and a missing file is a first run
+rather than an error. The directory field shows the directory in use: the default
 is filled in rather than described, and typing it back leaves the setting unset, so it goes on
 following the cache directory instead of pinning one path for good. It can be typed, or picked with the folder
 button beside it, which writes what it found into the same field -- the field stays the one value, and