feat: the host form, and the help window, are UI files

src/ui/source-form.ui is the form a source is added or edited with: an AdwDialog of
AdwPreferencesGroup rows (AdwEntryRow, AdwSpinRow, AdwSwitchRow) with its action in the header bar,
where the label says which mode the form is in -- Connect for a new host, Save for an existing
configuration. The grid of labels and bare entries it replaces had its hints in placeholders, which
vanish as soon as there is a value; they are the groups' descriptions now.

src/ui/help.ui is the help window's chrome: an AdwWindow with a header bar and an
AdwNavigationSplitView whose sidebar is the page list. The rows stay in C++ because the fourteen
pages are data, not template.

Four UI files now, 653 lines of template that used to be widget construction. Two libadwaita traps
cost a run each and are in docs/claude/invariants-gtk-and-chrome.md: an AdwDialog has no content
property (its child is a plain <child>, unlike an AdwToolbarView), and AdwEntryRow has no subtitle
(it is an AdwPreferencesRow, not an AdwActionRow). The unused form_field() helper went with them.

Every probe reading is unchanged -- [sources] form:, [sources] edit form:, [help] -- which is what
refactoring a shape rather than a behaviour should look like.

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

@@ -43,6 +43,11 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 
 ## Already done
 
+- `src/ui/source-form.ui` -- the form a source is added or edited with: an `AdwDialog` of
+  `AdwPreferencesGroup` rows, with its action in the header bar where the label says which mode it is in.
+- `src/ui/help.ui` -- the help window's chrome: an `AdwWindow` with a header bar and an
+  `AdwNavigationSplitView` whose sidebar is the page list. The rows stay in C++ because the fourteen
+  pages are data, not template.
 - `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
@@ -56,8 +61,7 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 
 | where | what | why it is the right next step |
 |---|---|---|
-| `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/source_dialog.cpp` (the chooser's own window) | the chooser: `.ui` for the window, its two pages and its action row. Its **host form is done**; what is left is the list, the notebook and the buttons around it | the last hand-built UI outside `ui.cpp`, and the file is already open |
 | `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 |
 
 Nothing in this list should be done in one pass. Each row is its own change, verified on its own.