Group member management: no way to promote a member to Organizer (only Co-Organizer)

## Bug On the group member-management UI, the `...` action menu for a member only offers two options: **Change to Co-Organizer** and **Remove**. There is **no** option to promote someone to the full **Organizer** role — and no equivalent "Transfer Organizer" flow either. Even when the viewer IS the group's Organizer (see screenshot: `alexis` is labeled `Ring Organizer`), the menu for other members only surfaces the Co-Organizer path. ## Roles visible in the UI - **Ring Organizer** (one per group, sole primary) - **Co-Organizer** - **Member** ## Expected One of these, depending on the intended model — **but something must exist**: ### If groups can have multiple Organizers Add a **Change to Organizer** option to the member action menu (visible only to existing Organizers). Co-Organizers should also be promotable to Organizer via the same menu. ### If there is exactly one Organizer per group (current design) Add a **Transfer Organizer role** flow. Existing Organizer initiates a transfer to another member; on confirmation, the current Organizer is demoted to Co-Organizer (or Member) and the target becomes the new Organizer. Standard one-owner-transfer UX. Either way, the current state is a dead end: the moment the single Organizer leaves or is unavailable, the group is stranded with only Co-Organizers and no path to elevate anyone. ## Reproduction 1. Log in as the Organizer of a group. 2. Go to the group's member management page. 3. Open the `...` menu on any non-Organizer member. 4. Observe: only **Change to Co-Organizer** and **Remove** appear. 5. Repeat on an existing Co-Organizer — same two options, still no way to promote them further. ## Impact - Single point of failure: if the sole Organizer leaves the platform or the org, no one can promote themselves or a Co-Organizer to take over. - Co-Organizers stay locked at that level forever. - Blocks normal succession / delegation workflows for groups, same issue pattern as Fider #71 (org Owner promotion). ## Related - **Fider #71** — same class of problem at the org level (`LAST_OWNER` guard blocks promoting anyone to Owner). Worth fixing both in one pass since the role-change logic likely shares code. - **Fider #68 / OP #5863** — "Ring creator not promoted to organizer; org owners have no permission to manage rings" — this bug is adjacent: even fixing auto-promotion on create doesn't help *existing* groups whose Organizer wants to hand off. ## Found by Manual testing