feat(pickers): a list that failed to load offers Retry

A picker whose options failed to load said so, but the only way back was closing its dialog.
Combobox takes `onRetry`: a Retry beside the message for the pointer, and Enter in the field for
the keyboard (the panel is a listbox, which may hold options only, so the alert names Enter for
assistive tech). Every caller with an error state passes its query's refetch: both user pickers,
the channel picker and the two Plex pickers.

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

@@ -20,6 +20,7 @@
   "demoSharedNotice": "You're in the shared demo account. Everything you do here — playlists, hidden videos, settings — is communal and may be changed by other demo visitors at the same time.",
   "backToTop": "Back to top",
   "retry": "Retry",
+  "retryWithEnter": "Press Enter to try again.",
   "loadError": "Couldn't load this — check your connection and try again",
   "seeAll": "See all",
   "scrollLeft": "Scroll left",

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

@@ -20,6 +20,7 @@
   "demoSharedNotice": "Egy közös demo fiókban vagy. Minden, amit itt csinálsz — lejátszási listák, elrejtett videók, beállítások — közös, és más demo látogatók egyszerre módosíthatják.",
   "backToTop": "Vissza a tetejére",
   "retry": "Újra",
+  "retryWithEnter": "Újrapróbáláshoz nyomd meg az Entert.",
   "loadError": "Nem sikerült betölteni — ellenőrizd a kapcsolatot, és próbáld újra",
   "seeAll": "Összes",
   "scrollLeft": "Görgetés balra",