chore: i18n time utils and use TFunction type (#6507) (#6539)

## Summary

Addresses two review comments from #6507:

- **`timeUtils.ts`** — route relative time strings (`just now`, `Xm
ago`, `Xh ago`, `Xd ago`) through i18n by accepting a `TFunction`
parameter and using new `time.relative.*` keys in `en-GB`
- **`ChatPanel.tsx`** — replace `ReturnType<typeof useTranslation>["t"]`
with `TFunction` from `i18next`

## Test plan
- [x] `task frontend:check` passes (695 tests)

frontend/editor/public/locales/en-GB/translation.toml

@@ -8151,6 +8151,12 @@ label = "Select a TSA server"
 [timestampPdf.steps]
 settings = "Settings"
 
+[time.relative]
+daysAgo = "{{count}}d ago"
+hoursAgo = "{{count}}h ago"
+justNow = "just now"
+minutesAgo = "{{count}}m ago"
+
 [tool]
 endpointUnavailable = "This tool is unavailable on your server."
 endpointUnavailableClickable = "Not available in this mode. Click to sign in."