feat(app): link liboakengine.dylib via frozen C ABI; UI fixes

- App no longer depends on the oakengine rlib: build.rs links the
  built liboakengine.dylib (+rpath, -export_dynamic, IOSurface) and
  src/oakui/ffi.rs declares the pure-C surface; RealEngine calls only
  the frozen oakengine_* C ABI
- host_syms.rs provides the oakcore_*/fb_* host symbols the dylib
  imports via dynamic lookup
- Fix Preferences dialog crash (spawn_modal reentrancy) with a
  regression test
- Timeline toolbar and viewer transport render C++-era icons (16px
  grid, dark/light themes) with localized tooltips
- i18n: complete en-US table, add untranslated-key detection test
- New dialogs module (preferences, export, progress)

examples/screenshot.rs

@@ -33,6 +33,7 @@
 use gpui::{px, size, AnyWindowHandle, AppContext, Result, VisualTestAppContext};
 use gpui_platform::current_platform;
 use oakapp::app::OakApp;
+use oakapp::oakui::MockEngine;
 
 const DEFAULT_WIDTH: f32 = 1600.0;
 const DEFAULT_HEIGHT: f32 = 900.0;
@@ -53,14 +54,24 @@ fn main() -> Result<()> {
 	cx.update(|app| app.init_colors());
 
 	let window = cx.open_offscreen_window(size(px(width), px(height)), |window, cx| {
-		cx.new(|cx| OakApp::new(window, cx))
+		cx.new(|cx| OakApp::<MockEngine>::new(window, None, cx))
 	})?;
 	let handle: AnyWindowHandle = window.into();
 
-	// Draw a few frames so the layout settles: the node editor fits its graph
-	// once the canvas size is known and the viewers upload their first CPU
-	// frame, both of which happen on the frame after the initial render.
-	for _ in 0..4 {
+	// Draw enough frames for the layout to settle and the async toolbar-icon
+	// assets to decode: the node editor fits its graph once the canvas size
+	// is known, the viewers upload their first CPU frame, and the PNG
+	// toolbar icons load through the background executor on the frame after
+	// the asset future resolves.
+	for _ in 0..16 {
+		cx.run_until_parked();
+		cx.update_window(handle, |_root, window, app| {
+			let _ = window.draw(app);
+		})?;
+	}
+	cx.run_until_parked();
+
+	for _ in 0..16 {
 		cx.run_until_parked();
 		cx.update_window(handle, |_root, window, app| {
 			let _ = window.draw(app);
@@ -69,6 +80,39 @@ fn main() -> Result<()> {
 	cx.run_until_parked();
 
 	let image = cx.capture_screenshot(handle)?;
+
+	// The timeline toolbar's tool icons (16px at 2× = 32px on 48px pitch)
+	// must render: the toolbar is the 31px row at the top of the bottom dock
+	// panel. Scan the bottom strip for the 8 tool cells and require most of
+	// them to contain bright glyph pixels, so a broken icon load fails the
+	// capture loudly instead of shipping an empty toolbar.
+	let th = height * 2.0;
+	let mut rendered = 0usize;
+	for (index, cell_x) in [24u32, 88, 152, 216, 280, 344, 408, 472].iter().enumerate() {
+		let mut bright = 0u32;
+		for dy in 0..80i32 {
+			for dx in 0..32i32 {
+				let x = (*cell_x as i32 + dx) as u32;
+				let y = (th as i32 - 320 + dy).max(0) as u32;
+				if x >= image.width() || y >= image.height() {
+					continue;
+				}
+				let p = image.get_pixel(x, y);
+				if p[0] > 150 && p[1] > 150 && p[2] > 150 {
+					bright += 1;
+				}
+			}
+		}
+		println!("[screenshot] toolbar tool {index} bright pixels: {bright}");
+		if bright > 20 {
+			rendered += 1;
+		}
+	}
+	assert!(
+		rendered >= 6,
+		"timeline toolbar icons did not render (only {rendered}/8 tool cells had pixels)"
+	);
+
 	std::fs::create_dir_all(std::path::Path::new(OUT).parent().unwrap())?;
 	image.save(OUT)?;
 	println!("wrote {OUT} ({}×{})", image.width(), image.height());