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.

src/ui/udpscope-ui.gresource.xml

@@ -11,5 +11,7 @@
   <gresource prefix="/org/udpscope/ui">
     <file preprocess="xml-stripblanks">shortcuts.ui</file>
     <file preprocess="xml-stripblanks">preferences.ui</file>
+    <file preprocess="xml-stripblanks">help.ui</file>
+    <file preprocess="xml-stripblanks">source-form.ui</file>
   </gresource>
 </gresources>