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/tauri.conf.json

@@ -13,10 +13,11 @@
     "windows": [
       {
         "title": "FlightTube",
-        "width": 1280,
-        "height": 840,
-        "minWidth": 940,
-        "minHeight": 600
+        "width": 1180,
+        "height": 780,
+        "minWidth": 900,
+        "minHeight": 560,
+        "center": true
       }
     ],
     "security": {