Make middle-click paste consistent across apps by enabling GTK/Chromium apps to receive selection buffer on button3 (#5577)

* Enable middle-click primary paste for GTK/Chromium apps

gsettings-desktop-schemas 50.x ships gtk-enable-primary-paste=false as
the default. GTK4 apps (Ghostty 1.3.0+) and Chromium-family browsers
honor that setting and silently no-op middle-click paste, even though
the primary selection itself works (wl-paste --primary returns text,
Firefox/LibreOffice paste fine because they implement primary paste
themselves rather than going through GTK).

Flip the GSetting to true so middle-click paste behaves the way Linux
desktop users expect across Ghostty, Brave, and Chromium.

Closes #5371

* Address Copilot review on #5577

Move install-time GSetting from install/config/ to install/first-run/
so it runs after login when a session D-Bus is guaranteed (matching the
existing gnome-theme.sh convention). Make the migration's gsettings
call best-effort with `|| true` so a missing dconf service doesn't
trigger the omarchy-migrate skip prompt.

migrations/1777929468.sh

@@ -0,0 +1,5 @@
+echo "Enable middle-click primary paste for GTK/Chromium-family apps (Ghostty, Brave, Chromium)"
+
+if command -v gsettings >/dev/null; then
+  gsettings set org.gnome.desktop.interface gtk-enable-primary-paste true || true
+fi