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.

docs/superpowers/plans/2026-10-11-ui-files-and-gtk4-practice.md

@@ -43,6 +43,10 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 
 ## Already done
 
+- `src/ui/preferences.ui` -- the preferences, as an `AdwPreferencesDialog` with `AdwEntryRow`,
+  `AdwSpinRow`, `AdwSwitchRow`, `AdwComboRow` and `AdwButtonRow` rows. The Apply button is gone: a row
+  applies as it changes, which is what those widgets are designed around, and `[prefs-probe]` reads the
+  file back after every change.
 - `src/ui/shortcuts.ui` -- the shortcut overview, with the mouse rows drawn as an icon cell.
 - The `src/ui/` GResource and its build block.
 - `[help]`'s `ui=`/`ui_accel=`/`ui_icon=`/`ui_bare=` reading, which checks the file against the tables
@@ -52,7 +56,6 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 
 | where | what | why it is the right next step |
 |---|---|---|
-| `src/ui_prefs.cpp` (~330 lines) | the preferences: `.ui` + `AdwPreferencesDialog`, `AdwEntryRow`, `AdwSpinRow`, `AdwSwitchRow`, `AdwComboRow` | the most visible win per line: it replaces four hand-built rows with four rows that look and behave like every other GNOME application's, and the file is self-contained |
 | `src/help_window.cpp` (351 lines) | the help window's chrome as a `.ui` (the pages themselves stay resources) | small, self-contained, and its probe already opens it |
 | `src/source_dialog.cpp` (~900 lines) | the source chooser and the host form: `.ui` + `AdwDialog`, with `AdwEntryRow`/`AdwSpinRow` for the form | the form is the second-biggest hand-built surface, and its probes drive every branch of it |
 | `src/ui.cpp` (5774 lines) | the main window's **skeleton** only -- the `AdwApplicationWindow`, the header bar, the plot area's placeholder, the panes' containers, the status bar -- with the custom widgets, popovers and dialogs left in code | one file, one window: converting the skeleton is reviewable, converting the whole file is not. Do it in slices (header bar, then the panes, then the trigger panel, then the left panes) with the probe suite after each |