feat: cross-team channels — external workspace identity badging

[emmpaul]

Part of the cross-team channels epic (#404). Sub-issue 4 of 4. Depends on sub-issues 1–3.

Summary

Make cross-team membership visible: members and message authors from the other workspace show a workspace-name badge inside a shared channel.

Locked decisions

• In a shared channel, users belonging to a different workspace than the viewer get a small workspace-name pill/badge on:
  • message author lines,
  • the member/roster list,
  • (where shown) hover cards / mentions.
• Purpose: an always-visible cue that the channel spans two workspaces, so people don't forget the audience is external.

Acceptance criteria

• Message authors from the other workspace render a workspace-name badge; same-workspace authors do not.
• The channel member list badges external members.
• Badging is driven by the owner/shared-team info exposed in the foundation DTO (sub-issue 1) — no extra per-message queries.
• i18n for any label copy (+ lang/fr.json); badge text is the workspace name (not translated) but any surrounding label is.
• Tests (incl. Pest browser test): external author/member badged, native author not. 100% coverage.

Out of scope

• Any behavior change beyond the visual cue.