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/claude/probe-readings.md

@@ -903,7 +903,7 @@ popover row reads `failed: <strerror>` — one stderr line, no retry storm again
 ## `[sources] prefs`
 
 ```
-[sources] prefs: opened=1 apply=1
+[sources] prefs: opened=1 rows=1
 [sources] prefs: dir=/tmp/udpscope-prefs-events keep=7 rel=1 theme=dark cfg_relative=1
 ```
 
@@ -1042,6 +1042,40 @@ library measures and draws the rect in tests of its own.
 
 ## `[prefs-probe]`
 
+```
+[prefs-probe] opened=1 rows=1 dir=/tmp/k5/cache/udpscope/events store=/tmp/k5/cache/udpscope/events/127.0.0.1-8000
+[prefs-probe] dir=/tmp/udpscope-prefs-events keep=7 rel=1 theme=dark live=1
+[prefs-probe] default restored: file=''
+[prefs-probe] browse: cancelled=1 icon_ok=1 field='/tmp/k5/cache/udpscope/events'
+```
+
+`UDPSCOPE_PROBE_PREFS_MS` opens the preferences, changes every row, and reads the settings file *back
+from disk* after each change -- not from `App::settings`, because a dialog that edits its own copy and
+never writes is exactly the failure this has to catch. **A change applies as it is made**: the rows are
+libadwaita's (`AdwEntryRow`, `AdwSpinRow`, `AdwSwitchRow`, `AdwComboRow`), and the `preferences.ui`
+dialog has no Apply button for the probe to press.
+
+- `opened=1 rows=1` is the dialog having come up *and* its rows having resolved by id. The file is
+  checked by the build (`xml-stripblanks`) and by `GtkBuilder` at load; a missing **id** is the one
+  mistake neither sees, and reading a null row on the first change is what it costs.
+- `dir` is the directory the *row* is offering -- the preference if there is one, otherwise the default
+  written out in full -- and `store` is the directory the cache is really using. **`store` must be `dir`
+  plus the source's folder name**, which is the reading that says the row shows the path in force rather
+  than a plausible string.
+- The second line must echo the probe's own input: directory `/tmp/udpscope-prefs-events`, keep `7`,
+  relative time `1`, theme `dark`. Any field that differs means the row, the write or the parse dropped
+  it. `live=1` is the theme having reached the running style manager rather than only the file.
+- `default restored: file=''` closes the loop from the other side: the probe puts the default path back
+  into the row, and the file must end up with **no** `event_cache_dir` at all. A path in the file is a
+  user's choice, and leaving the default is not one -- pinning it there would stop the setting following
+  the cache directory it is named after.
+- `browse: cancelled=1 icon_ok=1 field='...'` is the folder chooser. `cancelled=1` is its cancellable
+  having been cancelled, which is the *cancel* path a picker invites (a user's own Cancel button arrives
+  in the same callback with the same null answer); `icon_ok=1` is the icon theme knowing
+  `folder-open-symbolic`; and the field must still read the directory it read before -- **a cancelled
+  picker changes nothing**.
+
+
 ```
 [prefs-probe] opened=1 apply=1 dir=/tmp/k5/cache/udpscope/events store=/tmp/k5/cache/udpscope/events/127.0.0.1-8000
 [prefs-probe] dir=/tmp/udpscope-prefs-events keep=7 rel=1 theme=dark live=1