feat: intercept GDK button 8/9 for mouse back/forward navigation

WebKitGTK does not propagate XButton1/XButton2 (buttons 8 and 9) into
DOM events — event.button and event.buttons are always 0 for these
clicks. This prevents the frontend from detecting hardware back/forward
mouse buttons for history navigation.

Solution: patch Wails' window.c on Linux to intercept button-press-event
at the GTK signal level (before WebKit processes it). For button 8/9 we
call webkit_web_view_run_javascript() to dispatch a native CustomEvent
('verstak:navigate-back' / 'verstak:navigate-forward') into the page,
allowing the frontend to navigate history without any workaround on the
JS side.

The patch is applied automatically during build via scripts/build.sh:
  go mod vendor → patch -p0 < patches/window.c.button-press.patch
Vendor directory is gitignored.

scripts/build.sh

@@ -64,6 +64,25 @@ echo "  🧪 go test..."
 (cd "$ROOT" && go test -count=1 ./...)
 echo "  ✅ go test"
 
+# ── Mouse button patch ──
+# WebKitGTK does not propagate buttons 8/9 (XButton1/XButton2) into DOM events.
+# We patch Wails' window.c on Linux to intercept these GTK signals and dispatch
+# CustomEvent('verstak:navigate-back'/'verstak:navigate-forward') into the page.
+echo ""
+echo "[mouse-buttons]"
+PATCH_FILE="$ROOT/patches/window.c.button-press.patch"
+if [ -f "$PATCH_FILE" ]; then
+  echo "  📦 go mod vendor..."
+  (cd "$ROOT" && go mod vendor)
+  echo "  ✅ go mod vendor"
+  echo "  📝 applying window.c.button-press.patch..."
+  (cd "$ROOT" && patch -p0 --forward < "$PATCH_FILE" 2>/dev/null || true)
+  rm -f "$ROOT/vendor/github.com/wailsapp/wails/v2/internal/frontend/desktop/linux/window.c.rej"
+  echo "  ✅ window.c patched"
+else
+  echo "  ℹ️  patch file not found at $PATCH_FILE — skipping"
+fi
+
 # ── Wails ──
 echo ""
 echo "[wails]"