feat: add mpv to Multimedia Tools app list (#4940)

mpv has no official Windows installer; winget id shinchiro.mpv is the
de facto standard Windows build. Choco mpv/mpv.install are flagged
"Possibly broken" upstream, so winget is the sole install source.

config/applications.json

@@ -836,6 +836,14 @@
         "winget": "mpc-qt.mpc-qt",
         "foss": true
     },
+    "mpv": {
+        "category": "Multimedia Tools",
+        "content": "mpv",
+        "description": "mpv is a free, open source, and cross-platform media player supporting a wide variety of media formats, codecs, and subtitle types.",
+        "link": "https://mpv.io/",
+        "winget": "shinchiro.mpv",
+        "foss": true
+    },
     "matrix": {
         "category": "Communications",
         "choco": "element-desktop",