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.

todo.md

@@ -27,5 +27,7 @@
 10. [ ] move the interface into `.ui` resources, and to current GTK4/libadwaita widgets while doing it:
   - the shortcut overview is **done** (`src/ui/shortcuts.ui`, with the mouse gestures drawn as icons, and `[help]`'s `ui=` against `tables=` as the guard)
   - the preferences are **done** (`src/ui/preferences.ui`: `AdwPreferencesDialog` with `AdwEntryRow`/`AdwSpinRow`/`AdwSwitchRow`/`AdwComboRow`, applying as they change, no Apply button)
+  - the help window's chrome is **done** (`src/ui/help.ui`: `AdwWindow` + header bar + `AdwNavigationSplitView`)
+  - the host form is **done** (`src/ui/source-form.ui`); the source chooser's own window is next
   - next: the preferences (`AdwPreferencesDialog` + `AdwEntryRow`/`AdwSpinRow`/`AdwSwitchRow`), the help window's chrome, the source chooser and its host form, then the main window's skeleton
   - plan, conventions and acceptance for each slice: `docs/superpowers/plans/2026-10-11-ui-files-and-gtk4-practice.md`