Remove dead code found by full Periphery audit; add scan config + advisory CI (#1410)

Periphery 3.7.4 audit of both schemes (macOS + iOS, intersected so
platform-specific code is never touched), with test targets indexed and
the share extension built. 277 dead declarations removed or demoted:
dead forwarding wrappers (ChatViewModel+Nostr/+PrivateChat), removed-
feature remnants (autocomplete command suggestions, back-swipe tuning,
MediaSendError, GeohashParticipantTracker), unused Tor dormancy
bindings, assign-only properties, unused parameters (renamed to _), and
redundant public accessibility. 13 orphaned localization keys deleted
across all 29 locales (old pre-#1392 location-notes UI, app_info
warnings).

Two real tests were flagged as unused because they never ran: Swift
Testing methods missing @Test (NostrProtocolTests.
testAckRoundTripNIP44V2_Delivered, NotificationStreamAssemblerTests.
testAssemblesCompressedLargeFrame). Re-armed both; they pass.

Deliberately kept, now recorded in .periphery.baseline.json: iOS-only
code invisible to the CI macOS scan, C FFI signatures, keep-alive
NWPathMonitor reference, InboundEventKey.eventID (dedup semantics),
wifiBulk capability bit (reserved for Wi-Fi bulk work, used by
BitFoundation package tests), and the String secureClear cluster
(exercised by package tests).

New: .periphery.yml config and an advisory Dead Code CI job (mirrors
the SwiftLint precedent from #1361) that fails on findings not in the
committed baseline.

Verified: full macOS app suite, BitFoundation (119) and BitLogger (13)
package tests green; periphery scan --strict exits clean.

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

bitchatTests/ChatPrivateConversationCoordinatorContextTests.swift

@@ -100,11 +100,6 @@ private final class MockChatPrivateConversationContext: ChatPrivateConversationC
         unreadPrivateMessages.remove(peerID)
     }
 
-    func removePrivateChat(_ peerID: PeerID) {
-        privateChats.removeValue(forKey: peerID)
-        unreadPrivateMessages.remove(peerID)
-    }
-
     func migratePrivateChat(from oldPeerID: PeerID, to newPeerID: PeerID) {
         migratedChats.append((oldPeerID, newPeerID))
         guard oldPeerID != newPeerID, let source = privateChats[oldPeerID] else { return }
@@ -177,12 +172,10 @@ private final class MockChatPrivateConversationContext: ChatPrivateConversationC
     // Routing & acknowledgements
     private(set) var routedPrivateMessages: [(content: String, peerID: PeerID, messageID: String)] = []
     private(set) var routedReadReceipts: [(messageID: String, peerID: PeerID)] = []
-    private(set) var routedFavoriteNotifications: [(peerID: PeerID, isFavorite: Bool)] = []
     private(set) var meshReadReceipts: [(messageID: String, peerID: PeerID)] = []
     private(set) var geoPrivateMessages: [(content: String, recipientHex: String, messageID: String)] = []
     private(set) var geoDeliveryAcks: [(messageID: String, recipientHex: String)] = []
     private(set) var geoReadReceipts: [(messageID: String, recipientHex: String)] = []
-    private(set) var embeddedDeliveryAckMessageIDs: [String] = []
 
     func routePrivateMessage(_ content: String, to peerID: PeerID, recipientNickname: String, messageID: String) {
         routedPrivateMessages.append((content, peerID, messageID))
@@ -192,10 +185,6 @@ private final class MockChatPrivateConversationContext: ChatPrivateConversationC
         routedReadReceipts.append((receipt.originalMessageID, peerID))
     }
 
-    func routeFavoriteNotification(to peerID: PeerID, isFavorite: Bool) {
-        routedFavoriteNotifications.append((peerID, isFavorite))
-    }
-
     func sendMeshReadReceipt(_ receipt: ReadReceipt, to peerID: PeerID) {
         meshReadReceipts.append((receipt.originalMessageID, peerID))
     }
@@ -212,10 +201,6 @@ private final class MockChatPrivateConversationContext: ChatPrivateConversationC
         geoReadReceipts.append((messageID, recipientHex))
     }
 
-    func sendDeliveryAckViaNostrEmbedded(_ message: BitchatMessage, wasReadBefore: Bool, senderPubkey: String, key: Data?) {
-        embeddedDeliveryAckMessageIDs.append(message.id)
-    }
-
     // Favorites & notifications
     var favoriteRelationshipsByNoiseKey: [Data: FavoritesPersistenceService.FavoriteRelationship] = [:]
     private(set) var peerFavoritedUsUpdates: [(noiseKey: Data, favorited: Bool, nickname: String, nostrPublicKey: String?)] = []