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/components/NavBarMobile.jsx

@@ -1,4 +1,5 @@
 import * as React from 'react'
+import { useTranslation } from 'react-i18next'
 import Box from '@mui/joy/Box'
 import ListItemDecorator from '@mui/joy/ListItemDecorator'
 import Tabs from '@mui/joy/Tabs'
@@ -10,6 +11,7 @@ import Search from '@mui/icons-material/Search'
 import Person from '@mui/icons-material/Person'
 
 export default function NavBarMobile() {
+  const { t } = useTranslation('common')
   const [index, setIndex] = React.useState(0)
   const colors = ['primary', 'danger', 'success', 'warning']
   return (
@@ -32,7 +34,7 @@ export default function NavBarMobile() {
     >
       <Tabs
         size='lg'
-        aria-label='Bottom Navigation'
+        aria-label={t('bottomNav')}
         value={index}
         onChange={(event, value) => setIndex(value)}
         sx={theme => ({
@@ -98,7 +100,7 @@ export default function NavBarMobile() {
             <ListItemDecorator>
               <Person />
             </ListItemDecorator>
-            Profile
+            {t('profile')}
           </Tab>
         </TabList>
       </Tabs>