fix(client,core,server): stop admin channel actions from desyncing th…

[klobucar]

What this changes

Fixes a control-stream desync bug in the macOS client where admin channel
actions (and any user's profile update) corrupted the connection, plus a
server-side bug where channel ordering never worked despite the protocol
supporting it.

Why

The client's control-stream reader dispatched on the message-type byte but
never read the length-prefixed payload for CreateChannelResponse (0x40),
MetadataUpdateResponse (0x41 UpdateChannel / 0x42 UpdateProfile), or
AdminResponse (0x43 DeleteChannel) — they all fell into the unhandled
default case. That left the frame unread on the stream, so the next read
misinterpreted the previous frame's length prefix as a new message type and
corrupted control-stream parsing for the rest of the session. In practice:
any admin creating/updating/deleting a channel, or any user updating their
own profile, desynced their own connection, even though the action usually
succeeded server-side — the client just never found out and then misparsed
everything after.

Separately, create_channel_persistent always assigned position: 0 to new
channels (colliding with each other) and get_server_snapshot iterated a
DashMap with no sort, so the position field documented for "custom
ordering" in the protocol never actually ordered anything client-side.

How tested

• cargo test --workspace, cargo fmt --all -- --check, and
  cargo clippy --workspace --all-targets -- -D warnings all clean.
• macOS client builds (xcodebuild ... build succeeds) and
  AuraTests passes, scoped to that target (not the full scheme, per
  the UI-test Gatekeeper issue).
• Confirmed three AuraTests failures
  (MlsProtocolTests/testThreePartyMlsGroup,
  FuzzTests/testServerProfileWithRandomData,
  ConnectionRetryTests/testSavedConnectionParameters) are pre-exis
  reproducing them against unmodified main before this branch's ch
  not regressions from this PR.

Notes for the reviewer

• Added three new UniFFI records/decoders
  (CreateChannelResponseRecord, MetadataUpdateResponseRecord,
  AdminResponseRecord) in uniffi_bindings.rs and regenerated the
  bindings (gitignored, regenerate locally with `make generate-swift
  before building).
• A rejected admin action whose error message is exactly "Admin required"
  now clears the client's local isAdmin flag immediately, instead of
  leaving stale admin UI visible until reconnect — a small mitigatio
  the flag only being set once at login and never otherwise refreshed.
• Deliberately left out of scope, to avoid scope creep beyond "fix w
  broken": drag-to-reorder UI (ordering now works, but there's still no way
  to change order after creation), "create as Lobby" support, and
  admin-channel UI for the desktop (.NET) client, which currently has none
  at all.

Checklist

• Commit messages follow Conventional Commits
• Commits are signed off (git commit -s) per the DCO
• cargo fmt --all clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo test --workspace passes
• Wire-format / protocol changes have a corresponding update in no wire format changes (only decoding responses the protocol alreadydefines)
• Crypto changes have a corresponding update in docs/MLS_SECURIotocol_deviations.md — n/a