fix(downloads): media-library UI review fixes (E7)

Deep-review remediation on the frontend, all findings adversarially verified:

- LibraryPlayer picks audio vs video from the canonical media_kind (single source), not a
  hardcoded container whitelist, so an uploaded audio file with an uncommon/absent container
  renders in <audio> instead of a broken video box (legacy null media_kind keeps the heuristic).
- UploadDialog aborts the in-flight upload when the dialog is closed by X/ESC/backdrop/Back
  (not only the footer Cancel); drag highlight uses an enter/leave depth counter to stop the
  flicker; the progress bar gets a stable role=progressbar aria-label with the value on aria-valuenow.
- DownloadCenter's Shared tab renders honest loading/error states instead of a blank panel, and
  the library/shared row-action gating moves to a single tested helper (lib/downloadActions).

Tests: downloadActions gating matrix, the uploadReq XHR path (abort/401/422-localize/onerror),
and the media_kind-driven audio/video branch.

i18n: downloads.upload.uploadProgress (HU + EN).

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

@@ -33,6 +33,7 @@
     "namePlaceholder": "Leave blank to use the file name",
     "submit": "Upload",
     "uploading": "Uploading… {{pct}}%",
+    "uploadProgress": "Upload progress",
     "added": "File uploaded"
   },
   "tabs": {

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

@@ -33,6 +33,7 @@
     "namePlaceholder": "Hagyd üresen a fájlnév használatához",
     "submit": "Feltöltés",
     "uploading": "Feltöltés… {{pct}}%",
+    "uploadProgress": "Feltöltés folyamata",
     "added": "Fájl feltöltve"
   },
   "tabs": {