fix(downloads): confirm before clearing a link password; fix placeholder

Review follow-up on the share-link password UI:
- Removing a link's password makes it publicly watchable, a protection
  downgrade; gate it behind a danger-confirm like the neighbouring Revoke,
  spelling out that anyone with the URL can then watch without a password.
- When setting a first password the input reused the create-form "Password
  (optional)" placeholder, but it's required here (Save stays disabled until
  non-empty); use the "Set a password" wording instead.

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

@@ -123,7 +123,8 @@
     "changePassword": "Change password",
     "newPassword": "New password",
     "savePassword": "Save",
-    "removePassword": "Remove"
+    "removePassword": "Remove",
+    "removePasswordConfirm": "This makes the link public — anyone with the URL can watch it without a password."
   },
   "confirm": {
     "cancelTitle": "Cancel download?",

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

@@ -123,7 +123,8 @@
     "changePassword": "Jelszó módosítása",
     "newPassword": "Új jelszó",
     "savePassword": "Mentés",
-    "removePassword": "Törlés"
+    "removePassword": "Törlés",
+    "removePasswordConfirm": "Ezzel a link publikus lesz — jelszó nélkül bárki megnézheti, akinek megvan az URL."
   },
   "confirm": {
     "cancelTitle": "Megszakítod a letöltést?",