Add window-based target picker as the default; process list goes advanced

New host/src/inject/window_list.{hpp,cpp} enumerates visible, titled, non-tool
top-level (alt-tab-style) windows via EnumWindows -- root-owner only, our own
process excluded -- and maps each to its owning pid + image name. The Injection
panel now defaults to this window list (each row "title [process.exe pid]", with a
filter over title or process), since there are far fewer windows than processes and
a window maps straight to the HWND the capturer wants. The full process list stays
as the advanced picker under Debug details.

Verified live: launched the host and captured its window -- the picker lists real
windows (Discord/Firefox/Explorer/...) in "title [exe pid]" form, filter present,
and the host's own window correctly excluded. x64 build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/inject/window_list.hpp

@@ -0,0 +1,24 @@
+// Enumerates visible top-level windows for the injection target picker. There are
+// far fewer windows than processes, and a window maps directly to the HWND the
+// capturer wants, so this is the default (friendlier) way to pick a game.
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace coop
+{
+
+struct WindowEntry
+{
+	unsigned long pid = 0;    // owning process id
+	void* hwnd = nullptr;     // HWND (opaque here to keep windows.h out of the header)
+	std::wstring title;       // window caption
+	std::wstring exe_name;    // owning process image base name, e.g. "game.exe"
+};
+
+// Snapshot of the visible, titled, non-tool top-level (alt-tab-style) windows, with
+// our own process's windows excluded. Sorted by title (case-insensitive).
+std::vector<WindowEntry> list_windows();
+
+} // namespace coop