fix: ffmpeg version detection and player letterboxing

ffmpeg only accepts -version (single dash); it exits 8 on --version and
writes to stderr, so the status panel reported it missing even when
installed. Version flags are now per-tool and both streams are read.

The player used max-h-full inside a grid, which a 1080x1920 Short
overflowed. Switched to absolute fill with object-contain so portrait
and landscape both letterbox correctly.

Also sized the window to 1180x780 centered, which fits a laptop display.

src-tauri/src/commands.rs

@@ -80,29 +80,32 @@ fn bin(name: &str) -> String {
     name.to_string()
 }
 
-fn version_of(name: &str) -> Option<String> {
-    std::process::Command::new(bin(name))
-        .arg("--version")
+/// `flag` differs per tool: yt-dlp takes `--version`, ffmpeg only accepts
+/// `-version` (it exits non-zero on `--version` and writes to stderr), so the
+/// flag is passed in and both streams are consulted.
+fn version_of(name: &str, flag: &str) -> Option<String> {
+    let out = std::process::Command::new(bin(name))
+        .arg(flag)
         .output()
-        .ok()
-        .filter(|o| o.status.success())
-        .map(|o| {
-            String::from_utf8_lossy(&o.stdout)
-                .lines()
-                .next()
-                .unwrap_or("")
-                .trim()
-                .to_string()
-        })
-        .filter(|s| !s.is_empty())
+        .ok()?;
+    if !out.status.success() {
+        return None;
+    }
+    let text = if out.stdout.is_empty() {
+        String::from_utf8_lossy(&out.stderr).to_string()
+    } else {
+        String::from_utf8_lossy(&out.stdout).to_string()
+    };
+    let first = text.lines().next().unwrap_or("").trim().to_string();
+    (!first.is_empty()).then_some(first)
 }
 
 #[tauri::command]
 pub async fn check_prereqs(state: State<'_, AppState>) -> Result<Prereqs, String> {
     let library = state.library.lock().await.clone();
     Ok(Prereqs {
-        yt_dlp: version_of("yt-dlp"),
-        ffmpeg: version_of("ffmpeg").map(|v| {
+        yt_dlp: version_of("yt-dlp", "--version"),
+        ffmpeg: version_of("ffmpeg", "-version").map(|v| {
             // ffmpeg's first line is long; keep the useful head of it.
             v.split_whitespace().take(3).collect::<Vec<_>>().join(" ")
         }),