fix(ui): fix ID export for room owners and uncaptured memberships

[sanity]

Problem

Exporting an identity from any room other than the first showed "Cannot export: membership data not available" even when the user had sent messages. This happened because:

1. Room owners could never export — capture_self_membership_data() skips owners (returns early when verifying_key == owner_vk), so self_authorized_member was always None
2. Non-owners with uncaptured membership — if the member was pruned when capture_self_membership_data ran, or if room data predated the field, the export silently failed

Approach

Instead of requiring self_authorized_member to be pre-cached, the export modal now resolves the AuthorizedMember from three sources in priority order:

1. Cached self_authorized_member — existing fast path, no change
2. Owner self-sign — for room owners, creates a self-signed AuthorizedMember on the fly (owner signs themselves as invited_by: owner_id)
3. State lookup — for non-owners without cached data, finds the member in the current room_state.members and builds the invite chain

Also looks up member_info from current state as fallback when self_member_info is None.

Testing

• Added test_owner_self_signed_roundtrip to verify owner identity export/import works correctly (self-signed AuthorizedMember roundtrips, signature validates)
• Updated example data to populate self_authorized_member for "Member" rooms so the export feature is testable in example mode
• All existing identity tests pass (10/10)

Closes #187

[AI-assisted - Claude]