fix(gtk): register the gresource before startup so style.css loads

Ghostty's stylesheets (style.css, style-dark.css, style-hc*.css) were
never loaded. AdwApplication loads them from `resource-base-path` once,
during startup, and only if the resources exist at that moment. Zig's
gresource is registered by a glib-compile-resources C constructor before
main; the Rust build registered it lazily on first template instantiation
(windows, dialogs), i.e. after startup.

Visible effect: every rule in style.css was missing, e.g. the clipboard
confirmation dialog (`clipboard-read = ask`) rendered its preview without
the rounded inset and overlapping the "Remember choice" row.

Registering in Application::new restores parity; the Adwaita
"style-dark.css is deprecated" warnings Zig logs now appear too, which
confirms the files are picked up.

Test Plan:
- clipboard-read = ask, run `printf '\e]52;c;?\a'`: dialog is
  pixel-identical to Ghostty 1.3.1 (nested weston capture)

docs/rewrite/notes/ghostty-apprt-gtk.md

@@ -3,7 +3,8 @@
 ## Overview / status
 Port of ZIG/src/apprt/gtk (GTK4 + libadwaita, gtk4-rs 0.11 / libadwaita 0.9, baseline GTK 4.14 / adw 1.5).
 build.rs compiles resources/ui/**/*.blp with blueprint-compiler and bundles UI+CSS+icons into a gresource
-(`register_resources()`). Inspector/imgui deferred (menu action is a no-op with TODO). Dropped the ghostty-core
+(`register_resources()`, called in `Application::new` before startup so AdwApplication
+auto-loads style.css from `resource-base-path`). Inspector/imgui deferred (menu action is a no-op with TODO). Dropped the ghostty-core
 dependency (cycle): the core boundary is the trait set in `src/core.rs`.
 
 ## Core trait boundary (src/core.rs, authoritative spec = the file)