Add geohash bulletin board: persistent signed notices over mesh sync

New MessageType 0x23 carries TLV-encoded board posts and tombstones,
self-signed with the author's Ed25519 key ("bitchat-board-v1" /
"bitchat-board-del-v1" domains) so notices verify without the author
present. BoardStore persists raw signed packets under Application
Support/board/ (200 posts, 5 per author, oldest evicted; expiry sweep;
tombstones retained until the deleted post's original expiry) and is
wiped on panic.

Board packets join gossip sync as bit 8 of the existing variable-length
types bitfield (a second byte old decoders already accept and ignore),
with a 60s round and its own capacity, served straight from the board
store so retention has one owner. Posts relay like broadcasts; urgent
posts get the announce-class TTL cap.

UI: a pin button in the header opens the board for the current channel
(geohash board, or mesh-local board), with urgent-pinned newest-first
listing, compose with urgent toggle and 1/3/7-day expiry, and
swipe-delete on own posts. Geohash posts also publish one-way as
Nostr kind-1 location notes when relays are reachable.

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

bitchat/App/AppChromeModel.swift

@@ -18,6 +18,9 @@ final class AppChromeModel: ObservableObject {
     private let chatViewModel: ChatViewModel
     private var cancellables = Set<AnyCancellable>()
 
+    /// Bulletin-board coordinator, created on first use of the board sheet.
+    private(set) lazy var boardManager = BoardManager(transport: chatViewModel.meshService)
+
     init(chatViewModel: ChatViewModel, privateInboxModel: PrivateInboxModel) {
         self.chatViewModel = chatViewModel
         self.nickname = chatViewModel.nickname