i18n: extract shared strings into the existing common namespace

Part of #145.

Fourteen files whose user-facing strings are generic enough to belong in
`common` — loading and empty states, the confirmation modal, the shared
input modals (text/date/user/attachment viewer), the mobile nav bar, the
autocomplete input, the error screen and the file-upload error paths.

Extends the namespace that already exists, so `src/i18n/config.js` is
untouched and this cannot collide with any other extraction PR over the
`ns:` array. 28 keys added to `public/locales/en/common.json`.

English only — no translations, no behaviour change. Every t() value is
checked against this branch's base: the string must appear
character-for-character in the code it replaces (36 call sites).

One value is matched loosely and worth naming: `errorScreen.hideDetails`.
The base renders `{showDetails ? 'Hide' : 'Show'} error details`, so
neither full phrase exists contiguously in the source — only one branch of
the ternary can. Both keys hold exactly what each branch renders. The
sentence is kept whole rather than split around the ternary, because a
split sentence cannot be reordered by a translator.

src/views/Modals/Inputs/UserModal.jsx

@@ -1,8 +1,10 @@
 import { Avatar, Box, List, ListItem, Typography } from '@mui/joy'
 import ModalActions from '../../../components/common/ModalActions'
 import { useResponsiveModal } from '../../../hooks/useResponsiveModal'
+import { useTranslation } from 'react-i18next'
 
 const UserModal = ({ isOpen, performers = [], onSelect, onClose }) => {
+  const { t } = useTranslation('common')
   const { ResponsiveModal } = useResponsiveModal()
 
   return (
@@ -13,7 +15,7 @@ const UserModal = ({ isOpen, performers = [], onSelect, onClose }) => {
       fullWidth={true}
       title='Select User'
       footer={
-        <ModalActions secondary={{ label: 'Cancel', onClick: onClose }} />
+        <ModalActions secondary={{ label: t('cancel'), onClick: onClose }} />
       }
     >
       <List sx={{ mb: 2 }}>