Groups detail page still says "sequences" after the list page moved to "sightings"

#316 renamed the groups list's member count from "Sequences" to "Sightings" and carried the word through that page's copy (subtitle, popover, empty state, column tooltips), on the reasoning that a row is one recurring object and the rest of the app speaks in alerts and objects rather than sequences.

The group detail page was left untouched, so opening a row switches vocabulary mid-flow.

Strings still saying "sequence" in frontend/src/pages/SequenceGroupAnnotatePage.tsx

• :102 — remove-from-group confirm: Remove sequence #${member.sequence_id} from this group?
• :119 — member link title: "Open the per-sequence annotation page"
• :320 — legend: "Label — open any sequence below and label…"
• :324 — legend: "Validate — confirms every sequence shows…"
• :328 — legend: "Eject — ✕ removes a sequence that doesn't…"
• :385 — validate button title: "Confirms every sequence shows the same object and enables label propagation"

Scope note

"Sighting" is a new noun in this app — it exists only on the groups list today. Two coherent endings:

1. Carry "sighting" into the detail page (this ticket as written), so the groups flow is internally consistent.
2. Decide "sighting" was the wrong bet and revert the list page to "Sequences" — worse, since "sequence" is an internal data-model word users elsewhere never see.

The member id in the eject confirm is a real sequence id, so that string may want to keep the id while dropping the word (e.g. "Remove this sighting (sequence #123) from the object?"). Worth eyeballing rather than blind-replacing.

No behaviour change; copy only. Tests asserting these strings live in frontend/tests/pages/ (the list-page equivalents were updated in #316).

Start in frontend/src/pages/SequenceGroupAnnotatePage.tsx at the listed strings around lines 102, 119, 320-328, and 385. Review the related assertions in frontend/tests/pages/ and update the detail-page copy consistently, preserving the real sequence id where appropriate. Done means the groups flow uses the agreed sighting vocabulary and its copy tests pass.