feat(hub): name a group's host on its card, not its visibility

"private" and "invite" were on every card because they are the default for
every group on the hub — two badges that never varied and so never told
anyone anything. Who hosts a group does vary, and it is the thing that tells
two groups of the same name apart: the hub enforces name uniqueness per owner
account, not globally.

- The "My groups" cards (app.js) drop both badges and render the name through
  GroupName, which puts the owner beside it as a smaller "@handle" — the same
  way the group header and Explore already write it. "admin" stays; it varies.
  /v1/groups/mine already returns owner_username, so no API change.
- Explore drops its join_policy badge for the same reason. An open group still
  announces itself through the Join button, which is the useful half, and its
  @host was already there.
- .group-card h3 .gn-owner is set to 0.8em: a card's h3 is reset to 1em, so
  the 0.55em meant for a page heading rendered the handle at about half the
  body size — smaller than the name, as it should be, but past readable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8EDjk6pkYZrCbo63m2x87

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