fix: row titles mean their mnemonics instead of drawing the underscore

Fifteen display strings in the two preference forms were written as GTK mnemonics -- `_Directory`,
`_Server`, `Multicast _group`, `_Theme` and the rest -- but the property that makes GTK read them was
never set, so GTK drew the underscore literally: `_Directory` in the row, not a D with a line under it.

Two switches are involved, FALSE by default in each case. `GtkLabel`/`GtkButton` take `use-underline`;
the rows take `AdwPreferencesRow:use-underline`, "whether an embedded underline in the title indicates a
mnemonic". The two buttons in the source form already set it (so their mnemonics worked and showed no
underscore); the thirteen rows did not. All thirteen now do.

The row classes that carry the property are not all related -- AdwEntryRow has its own, while AdwSpinRow,
AdwComboRow and AdwSwitchRow get it through AdwActionRow -- so the probe reads it by name on a GObject
(g_object_class_find_property, then g_object_get) rather than casting to one row type; a cast to a
`GtkWidget*` where the type is only forward-declared does not compile at all.

`_Cancel` and `_Connect` both claimed C in the same form, so Alt+C could only reach one of them: that
marker was unreachable, which is worse than absent because it is in the source to be trusted. `_Connect`
becomes `Conn_ect`, freeing C for Cancel.

[prefs-probe] reads the count (4/4) on the real widgets; check_ui validates the property against the GIR
for every object in every .ui file, inherited properties included.

docs/claude/invariants-gtk-and-chrome.md

@@ -280,3 +280,21 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   re-enter a dead window. GTK does not promise to take a transient down with its parent, and a window
   whose handlers point into a returned stack frame is how this project already earned one
   use-after-free.
+
+- **A `_` in a label is a mnemonic only on a widget that was asked for one.** GTK reads the underscore as
+  "the next letter is this widget's accelerator, and draw it underlined", so wherever the widget has not
+  been asked, GTK draws the underscore *literally* -- which is what `_Directory`, `_Server`, `Keep _events`
+  and eleven more titles did in the two preference forms, for as long as they existed. Two different
+  switches, FALSE by default in both cases: `GtkLabel`/`GtkButton` take `use-underline`, and the rows take
+  **`AdwPreferencesRow:use-underline`** ("whether an embedded underline in the title indicates a mnemonic").
+  The row classes that carry it are not all related -- `AdwEntryRow` has its own, while
+  `AdwSpinRow`/`AdwComboRow`/`AdwSwitchRow` get it through `AdwActionRow` -- so `row_underlines()` asks by
+  name on a `GObject` (`g_object_class_find_property`, then `g_object_get`) instead of casting to one row
+  type: casting a row whose type is only forward-declared in that translation unit does not even compile.
+  `test-ui` is what catches the property being set on a class that does not have it -- it validates every
+  property against the GIR, inherited ones included.
+- **A mnemonic is unique per window, and the first match wins.** `_Cancel` and `_Connect` both claimed C in
+  one form, so `Alt+C` could only ever reach one of them and the other's marker was unreachable -- worse
+  than no mnemonic, because the underscore is there in the source for a reader to trust. `Conn_ect` freed
+  it. Rows on different pages of a preferences dialog may share a letter: an unshown page cannot be
+  activated.