feat(electrobun): native C resize library (Wails pattern)

New: agor-pty/native/agor_resize.c — C shared library that:
- Connects button-press-event on GtkWindow + WebView children in C
- Stores mouseButton, xroot, yroot, dragTime from real GTK events
- Exports agor_resize_start(edge) for Bun FFI

Build: gcc -shared -fPIC -o libagor-resize.so agor_resize.c $(pkg-config --cflags --libs gtk+-3.0)

New: ui-electrobun/src/bun/native-resize.ts — Bun FFI wrapper
App.svelte: simplified to just edge detection + RPC call

agor-pty/native/agor_resize.c

@@ -0,0 +1,134 @@
+/**
+ * libagor-resize.so — Native GTK resize handler for Electrobun.
+ *
+ * Connects button-press-event on the GtkWindow in C (not JSCallback).
+ * Stores mouse state from the real GTK event. Exports start_resize(edge)
+ * for Bun FFI to call — uses the stored event data for begin_resize_drag.
+ *
+ * This is the Wails pattern adapted for Electrobun/Bun.
+ *
+ * Build: gcc -shared -fPIC -o libagor-resize.so agor_resize.c \
+ *        $(pkg-config --cflags --libs gtk+-3.0)
+ */
+
+#include <gtk/gtk.h>
+#include <gdk/gdkx.h>
+#include <stdio.h>
+
+/* Stored mouse state from the last button-press event */
+static gdouble stored_xroot = 0.0;
+static gdouble stored_yroot = 0.0;
+static guint32 stored_time = 0;
+static guint stored_button = 1;
+static GtkWindow *stored_window = NULL;
+
+/* Border width for hit-test (pixels) */
+static int border_width = 8;
+
+/**
+ * GTK button-press-event handler — stores mouse state for later use.
+ * Connected directly in C — no JSCallback boundary crossing.
+ * Returns FALSE to let the event propagate to WebKitGTK normally.
+ */
+static gboolean
+on_button_press(GtkWidget *widget, GdkEventButton *event, gpointer user_data)
+{
+    if (event == NULL) return FALSE;
+    if (event->button == 3) return FALSE;  /* Skip right-click */
+
+    if (event->type == GDK_BUTTON_PRESS && event->button == 1) {
+        stored_xroot = event->x_root;
+        stored_yroot = event->y_root;
+        stored_time = event->time;
+        stored_button = event->button;
+    }
+    return FALSE;  /* Propagate — WebKitGTK still processes the event */
+}
+
+/**
+ * Initialize: connect button-press-event on the GtkWindow.
+ * Call ONCE after window creation from Bun FFI.
+ *
+ * @param window_ptr  GtkWindow* (from Electrobun's mainWindow.ptr)
+ * @param border      Border width in pixels for edge detection
+ */
+void agor_resize_init(void *window_ptr, int border)
+{
+    if (!GTK_IS_WINDOW(window_ptr)) {
+        fprintf(stderr, "[agor-resize] ERROR: not a GtkWindow: %p\n", window_ptr);
+        return;
+    }
+
+    stored_window = GTK_WINDOW(window_ptr);
+    border_width = border > 0 ? border : 8;
+
+    /* Add event masks */
+    gtk_widget_add_events(GTK_WIDGET(stored_window),
+        GDK_POINTER_MOTION_MASK |
+        GDK_BUTTON_PRESS_MASK |
+        GDK_BUTTON1_MOTION_MASK);
+
+    /* Connect button-press on the window — fires even when WebView covers everything
+       because we also connect on the WebView child below */
+    g_signal_connect(stored_window, "button-press-event",
+                     G_CALLBACK(on_button_press), NULL);
+
+    /* Walk down to find the WebView and connect there too */
+    GtkWidget *child = gtk_bin_get_child(GTK_BIN(stored_window));
+    while (child && GTK_IS_BIN(child)) {
+        g_signal_connect(child, "button-press-event",
+                         G_CALLBACK(on_button_press), NULL);
+        child = gtk_bin_get_child(GTK_BIN(child));
+    }
+
+    fprintf(stderr, "[agor-resize] Initialized: window=%p border=%d\n",
+            window_ptr, border_width);
+}
+
+/**
+ * Start a resize drag using the stored mouse state.
+ * Call from Bun FFI when JS detects a mousedown in the border zone.
+ *
+ * @param edge  GdkWindowEdge value (0=NW, 1=N, 2=NE, 3=W, 4=E, 5=SW, 6=S, 7=SE)
+ * @return 1 on success, 0 on failure
+ */
+int agor_resize_start(int edge)
+{
+    if (!stored_window) {
+        fprintf(stderr, "[agor-resize] ERROR: not initialized\n");
+        return 0;
+    }
+    if (edge < 0 || edge > 7) {
+        fprintf(stderr, "[agor-resize] ERROR: invalid edge %d\n", edge);
+        return 0;
+    }
+    if (stored_time == 0) {
+        fprintf(stderr, "[agor-resize] ERROR: no stored event (click first)\n");
+        return 0;
+    }
+
+    fprintf(stderr, "[agor-resize] begin_resize_drag edge=%d btn=%u xy=(%.0f,%.0f) t=%u\n",
+            edge, stored_button, stored_xroot, stored_yroot, stored_time);
+
+    gtk_window_begin_resize_drag(
+        stored_window,
+        (GdkWindowEdge)edge,
+        stored_button,
+        (gint)stored_xroot,
+        (gint)stored_yroot,
+        stored_time
+    );
+
+    return 1;
+}
+
+/**
+ * Get the stored mouse position (for debugging).
+ */
+void agor_resize_get_state(double *out_x, double *out_y, unsigned int *out_time, unsigned int *out_button)
+{
+    if (out_x) *out_x = stored_xroot;
+    if (out_y) *out_y = stored_yroot;
+    if (out_time) *out_time = stored_time;
+    if (out_button) *out_button = stored_button;
+}