skeleton: GtkApplication, window, GSettings schema, desktop, tests (ft0)

M0 (task ft0): the app shell — no feature views yet (those are M1 large, M7
grid).

- src/main.c: g_application_run + --version via handle-local-options (--help
  is GApplication's own); exits before activate so --version works headless.
- src/app.{c,h}: GgazeApp : AdwApplication : GtkApplication (libadwaita
  styling, decision #29), single-instance (decision #32),
  G_APPLICATION_HANDLES_OPEN; activate + open reuse the active window
  (replace-on-new-open); ggaze_window_open remembers the GFile.
- src/window.{c,h}: GgazeWindow : GtkApplicationWindow with AdwHeaderBar and a
  GtkStack (children 'grid'/'large', default 'grid'); dispose clears the file.
- data/org.buetow.ggaze.desktop (image/* MIME, validated) and
  data/org.buetow.ggaze.gschema.xml (all keys + defaults, --strict clean).
- meson: libggaze static lib shared by the executable and tests; install to
  bindir/applications/glib-2.0 schemas; gnome.post_install compiles schemas +
  updates desktop db.
- tests: unit test_app (--version/--help/unknown via subprocess), integration
  test_window (offscreen stack checks + open-titles), bootstrap. 3/3 green.
- docs/tech-stack.md: window-geometry shape corrected to (width,height,
  fullscreen,maximized) to match the schema.

1 files changed, 74 insertions, 0 deletions

diff --git a/src/app.c b/src/app.c
new file mode 100644
index 0000000..587feb7
--- /dev/null
+++ b/src/app.c
@@ -0,0 +1,74 @@
+/*:*
+ * ggaze — application object
+ *
+ * Implements GgazeApp (GgazeApp : AdwApplication : GtkApplication). Wires the
+ * GApplication `open` signal (file or folder) and the default `activate`
+ * (no-arg) path to a GgazeWindow, reusing the active window for the
+ * single-instance "replace on new open" behaviour (decision #32). Real
+ * file-vs-folder resolution is the navigator's job in M2; M0 just remembers
+ * the path and titles the window. See docs/architecture.md.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include "app.h"
+#include "window.h"
+#include "ggaze-config.h"
+
+#include <glib.h>
+
+struct _GgazeApp {
+   AdwApplication parent_instance;
+};
+
+G_DEFINE_TYPE(GgazeApp, ggaze_app, ADW_TYPE_APPLICATION)
+
+static GgazeWindow *
+_ensure_window(GgazeApp *p_app) {
+   GtkWindow *p_active =
+      gtk_application_get_active_window(GTK_APPLICATION(p_app));
+   if (p_active != NULL && GGAZE_IS_WINDOW(p_active)) {
+      return (GgazeWindow *)p_active;
+   }
+   return (ggaze_window_new(p_app));
+}
+
+static void
+ggaze_app_activate(GApplication *p_app) {
+   GgazeWindow *p_win = _ensure_window(GGAZE_APP(p_app));
+   gtk_window_present(GTK_WINDOW(p_win));
+}
+
+static void
+ggaze_app_open(GApplication *p_app, GFile **p_files, gint n_files,
+               const gchar *c_hint) {
+   (void)c_hint;
+   GgazeApp    *p_app_self = GGAZE_APP(p_app);
+   GgazeWindow *p_win      = _ensure_window(p_app_self);
+   /* open-question Z / decision #27: one file opens it; many files open the
+    * first file's folder with the first current (navigator wiring in M2). */
+   if (n_files > 0 && p_files != NULL) {
+      ggaze_window_open(p_win, p_files[0]);
+   }
+   gtk_window_present(GTK_WINDOW(p_win));
+}
+
+static void
+ggaze_app_init(GgazeApp *p_app) {
+   (void)p_app;
+}
+
+static void
+ggaze_app_class_init(GgazeAppClass *p_klass) {
+   GApplicationClass *p_app_class = G_APPLICATION_CLASS(p_klass);
+   p_app_class->activate          = ggaze_app_activate;
+   p_app_class->open              = ggaze_app_open;
+}
+
+GgazeApp *
+ggaze_app_new(void) {
+   return (
+      GGAZE_APP(g_object_new(GGAZE_TYPE_APP, "application-id", GGAZE_APP_ID,
+                             "flags", G_APPLICATION_HANDLES_OPEN, NULL)));
+}
\ No newline at end of file