fix(ui): allow setting a transcoding Default Bit Rate of 0

The Default Bit Rate dropdown on the Transcoding create/edit forms was fed
BITRATE_CHOICES, which starts at 32. There was no way to pick 0, and the
SelectInput was not resettable, so an admin could neither create nor restore a
transcoding with no default bit rate, such as the default FLAC one (seeded with
0 in consts.DefaultTranscodings). Editing that row also rendered a blank
dropdown, since its stored value matched no choice.

Adds TRANSCODING_BITRATE_CHOICES, which prepends a 0 entry labelled 'None' to
the shared list. The forms use it as SelectInput choices, and the list and
read-only show view render it through SelectField, so all four screens resolve
the label from the same array and cannot drift. The shared BITRATE_CHOICES is
left untouched, because 0 is not a meaningful option for the player Max. Bit
Rate or the share dialog.

Reported in discussion #6107, where a user had deleted the default
transcodings and could not recreate the FLAC one.

ui/src/transcoding/TranscodingList.jsx

@@ -1,7 +1,8 @@
 import React from 'react'
-import { Datagrid, TextField } from 'react-admin'
+import { Datagrid, SelectField, TextField } from 'react-admin'
 import { useMediaQuery } from '@material-ui/core'
 import { SimpleList, List } from '../common'
+import { TRANSCODING_BITRATE_CHOICES } from '../consts'
 import config from '../config'
 
 const TranscodingList = (props) => {
@@ -16,13 +17,22 @@ const TranscodingList = (props) => {
         <SimpleList
           primaryText={(r) => r.name}
           secondaryText={(r) => `format: ${r.targetFormat}`}
-          tertiaryText={(r) => r.defaultBitRate}
+          tertiaryText={(r) => (
+            <SelectField
+              record={r}
+              source="defaultBitRate"
+              choices={TRANSCODING_BITRATE_CHOICES}
+            />
+          )}
         />
       ) : (
         <Datagrid rowClick={config.enableTranscodingConfig ? 'edit' : 'show'}>
           <TextField source="name" />
           <TextField source="targetFormat" />
-          <TextField source="defaultBitRate" />
+          <SelectField
+            source="defaultBitRate"
+            choices={TRANSCODING_BITRATE_CHOICES}
+          />
           <TextField source="command" />
         </Datagrid>
       )}