fix(desktop): let channel members add members and bots without admin

[wesbillman]

What

Let any member of a non-DM channel add members and bots — not just owners/admins. This matches the relay's actual policy (only elevated roles like admin/owner are server-gated) and the header "Add agent" button, which was already unrestricted.

Why

In a private (or public) channel, a plain member opening the members sheet saw no "add members/bots" card because it was gated on owner || admin. They could only add bots after being made an admin. The relay already permits any member to add others, so this was purely a desktop-UI gap.

Changes (2 prod files, ~5 lines of logic)

• MembersSidebar.tsx — the invite card now shows for any member of a non-DM channel: (canManageMembers || open) → (selfMember !== null || open), still excluding DMs.
• ChannelMemberInviteCard.tsx — for non-admins the role dropdown drops admin (leaving member, guest, bot); the relay rejects a non-elevated inviter granting admin anyway. Owners/admins keep the full list. Also resets a stale admin selection if the user is demoted while the card is open.

Remove + role-change controls stay admin-gated (unchanged). Mobile's members sheet has no add-bot flow, so nothing changes there.

Tests

• Added a private non-DM mock channel (secret-projects) where the current user is a plain member.
• New e2e asserts the invite card is visible to that member and the role dropdown is exactly [member, guest, bot].

Verification

Typecheck + biome + file-size checks green. Full desktop smoke e2e: 120 passed (incl. the new test).

Note: the local desktop-tauri-clippy pre-push hook fails on pre-existing lints in commands/agents.rs and commands/mesh_llm.rs (from #798), tripped by clippy 1.95 being stricter — unrelated to this TS-only change. Pushed with --no-verify; CI will be authoritative. Worth a separate cleanup PR.