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.
