feat(render): fullscreen headless receiver with letterboxing

A headless receiver has no window manager, so a windowed window is
meaningless there — the screencast should simply fill the screen. The
renderer now goes borderless fullscreen automatically when SDL runs
the KMSDRM backend, and --fullscreen forces the same behavior in
desktop sessions. The video keeps its aspect ratio via
SDL_SetRenderLogicalPresentation(LETTERBOX): a 4:3 desktop on a 16:9
TV renders with black bars instead of a stretched picture, the cursor
is hidden, and the clear-before-draw keeps the bars black.

Verified live on a 3440x1440 monitor: the --fullscreen receiver window
covered the entire display while a 4:3 synthetic feed rendered with
pure-black pillarbox bars (Y=0/SAT=0) and a saturated-red center
(V=254). meson test 5/5 in both build configurations.

src/app/cli.cpp

@@ -9,7 +9,7 @@ namespace {
 
 void print_usage() {
     std::fputs("usage: screencast --send [--target monitor|window] [--peer HOST[:PORT]] [--bitrate KBPS]\n"
-               "       screencast --receive [--port PORT] [--signaling-port PORT]\n"
+               "       screencast --receive [--port PORT] [--signaling-port PORT] [--fullscreen]\n"
                "       screencast --discover [--timeout SECONDS]\n"
                "\n"
                "--send without --peer discovers a receiver on the LAN and requires\n"
@@ -110,6 +110,8 @@ std::optional<Command> parse_cli(int argc, const char* const argv[]) {
                 return std::nullopt;
             }
             receive.signaling_port = port;
+        } else if (argument == "--fullscreen") {
+            receive.fullscreen = true;
         } else if (argument == "--timeout") {
             std::string_view value;
             if (!next_argument(argc, argv, index, value) || !parse_int(value, discover.timeout_seconds) ||