feat(downloads): in-app player for library items (E7 S4b)

Play a completed library item in-app: a Play button on Library rows (and
Shared rows with download access) opens a modal that streams the local file
into a native <video controls> (or <audio> for audio-only assets).

Rather than the YouTube IFrame PlayerModal or the Plex HLS PlexPlayer — both
bound to their own domain APIs and unable to point at a local file — this
reuses the shared Modal a11y shell (focus-trap, ESC, layer-stack, portal) and
the range-aware /api/downloads/{id}/file endpoint. S4a's universal-codec
guarantee (mp4/H.264/AAC + m4a/mp3) makes native playback always work, so seek,
volume, fullscreen and speed come for free with zero transcode.

frontend/src/i18n/locales/en/downloads.json

@@ -56,6 +56,7 @@
     "delete": "Remove",
     "retry": "Retry",
     "download": "Save to my device",
+    "play": "Play",
     "rename": "Rename",
     "editDetails": "Edit details",
     "share": "Share (copy link)",
@@ -64,6 +65,9 @@
     "redownload": "Re-download (rebuild file)",
     "removeShared": "Remove from my list"
   },
+  "player": {
+    "unsupported": "Your browser can't play this file."
+  },
   "empty": {
     "queue": "Nothing in the queue.",
     "done": "No downloads yet.",

frontend/src/i18n/locales/hu/downloads.json

@@ -56,6 +56,7 @@
     "delete": "Eltávolítás",
     "retry": "Újra",
     "download": "Mentés a gépemre",
+    "play": "Lejátszás",
     "rename": "Átnevezés",
     "editDetails": "Adatok szerkesztése",
     "share": "Megosztás (link másolása)",
@@ -64,6 +65,9 @@
     "redownload": "Újraletöltés (fájl újraépítése)",
     "removeShared": "Eltávolítás a listámból"
   },
+  "player": {
+    "unsupported": "A böngésződ nem tudja lejátszani ezt a fájlt."
+  },
   "empty": {
     "queue": "A sor üres.",
     "done": "Még nincs letöltés.",