Private groups: creator-managed encrypted group chat over the mesh (#1383)

* Add capability bits to announce TLV

Announces now carry an optional capabilities TLV (0x05): a little-endian
bitfield with named bits for upcoming features (prekeys, wifiBulk,
gateway, groups, board, vouch, meshDiagnostics). Old clients skip the
unknown TLV; peers without it decode as nil so features can distinguish
"legacy peer" from "advertises nothing".

PeerCapabilities lives in BitFoundation with a minimal-length encoding
that preserves unknown bits for forward compatibility. Peer capabilities
are stored in the BLE peer registry on verified announce and exposed via
BLEService.peerCapabilities(_:). The local advertisement set is empty
until each feature ships its bit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Private groups: creator-managed encrypted group chat over the mesh

Small encrypted crews (hard cap 16) between public broadcast and 1:1 DMs:

Protocol
- MessageType.groupMessage = 0x25: broadcast packets with a cleartext
  16-byte group ID + epoch, ChaCha20-Poly1305 ciphertext (epoch bound as
  AEAD AAD), inner Ed25519 sender signature over
  "bitchat-group-msg-v1"|groupID|messageID|timestamp|content
- NoisePayloadType.groupInvite = 0x06 / .groupKeyUpdate = 0x07:
  creator-signed group state (key, epoch, roster) 1:1 over Noise; signature
  over "bitchat-group-v1"|groupID|epoch|key-hash|roster-hash and the Noise
  session peer must BE the creator
- SyncTypeFlags bit 10 (groupMessage): variable-length LE bitfield widens
  1 -> 2 bytes inside the length-prefixed REQUEST_SYNC TLV; old clients
  ignore unknown bits and answer with types they know
- PeerCapabilities.localSupported now advertises .groups

Storage
- GroupStore: symmetric keys in the keychain, roster/name/epoch as
  protected JSON in Application Support; wiped in panicClearAllData()

Behavior
- Non-members relay 0x25 like any broadcast but cannot read it; group
  messages join gossip-sync backfill with the public-message window
- Receivers drop wrong-epoch envelopes, bad sender signatures, and
  senders missing from the creator-signed roster
- Fire-and-flood delivery (no per-member acks in v1)

UI
- Groups open as chat windows through the private-chat sheet (virtual
  "group_" peer IDs); groups section in the people sheet; /group
  create/invite/remove/leave/list commands; invitees get a system message
  + notification and the group appears in their people sheet

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Private groups: fix TLV truncation, roster downgrade, removal notice, block, media, signable bytes

Addresses the Codex review and adversarial-review findings on #1383:

- TLV encoding now throws GroupTLVError.valueTooLong instead of clamping to
  65535 and truncating, so an oversize group message fails to seal and
  surfaces send_failed rather than shipping ciphertext recipients drop.
- Roster nicknames truncate on a Character boundary (never mid-scalar), so a
  multi-byte nickname can no longer make the whole signed roster undecodable.
- Invites now bump the epoch (rotate the key) like removals, giving every
  roster change a strictly-increasing epoch so out-of-order invite states no
  longer last-writer-wins a just-added member back out.
- Removing a member now sends them a creator-signed roster-without-them under
  a throwaway all-zero key (never the rotated key), so their client
  deactivates the group and surfaces "removed" instead of going silently dark.
- /block is enforced in the group receive path: a blocked member's messages
  are dropped from display and notifications, consistent with every other
  inbound path.
- Media affordances are disabled in group chats (both computed sites) so the
  composer can't strand a media placeholder that never sends; media-in-groups
  is a documented v2 item.
- Creator signature now covers the group name and the sender signature covers
  the epoch (wire-format-affecting; needs Android parity before ship).
- Explicit isGroup guard in markPrivateMessagesAsRead so read/delivered
  receipts can never leak into group conversations under a future refactor.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

bitchat/App/PrivateConversationModels.swift

@@ -108,7 +108,13 @@ struct PrivateConversationHeaderState: Equatable {
     let encryptionStatus: EncryptionStatus?
 
     var supportsFavoriteToggle: Bool {
-        !conversationPeerID.isGeoDM
+        !conversationPeerID.isGeoDM && !conversationPeerID.isGroup
+    }
+
+    /// Group chats have no single peer identity behind the header: no
+    /// fingerprint screen, no per-peer encryption badge.
+    var isGroupConversation: Bool {
+        conversationPeerID.isGroup
     }
 }
 
@@ -206,6 +212,13 @@ final class PrivateConversationModel: ObservableObject {
             }
             .store(in: &cancellables)
 
+        chatViewModel.groupStore.$groups
+            .receive(on: DispatchQueue.main)
+            .sink { [weak self] _ in
+                self?.refreshSelectedConversation()
+            }
+            .store(in: &cancellables)
+
         NotificationCenter.default.publisher(for: Notification.Name("peerStatusUpdated"))
             .receive(on: DispatchQueue.main)
             .sink { [weak self] _ in
@@ -229,6 +242,26 @@ final class PrivateConversationModel: ObservableObject {
     }
 
     private func makeHeaderState(for conversationPeerID: PeerID) -> PrivateConversationHeaderState {
+        // Group chats: the "peer" is the whole crew. Name + member count in
+        // the header; availability reads as mesh since group traffic floods
+        // the local mesh, and the per-peer encryption badge does not apply.
+        if conversationPeerID.isGroup {
+            let displayName: String
+            if let group = chatViewModel.groupStore.group(for: conversationPeerID) {
+                displayName = "#\(group.name) (\(group.members.count))"
+            } else {
+                displayName = String(localized: "common.unknown", comment: "Fallback label for unknown peer")
+            }
+            return PrivateConversationHeaderState(
+                conversationPeerID: conversationPeerID,
+                headerPeerID: conversationPeerID,
+                displayName: displayName,
+                availability: .meshReachable,
+                isFavorite: false,
+                encryptionStatus: nil
+            )
+        }
+
         let headerPeerID = chatViewModel.getShortIDForNoiseKey(conversationPeerID)
         let peer = chatViewModel.getPeer(byID: headerPeerID)
         let displayName = resolveDisplayName(for: conversationPeerID, headerPeerID: headerPeerID, peer: peer)