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/Performance/PerformanceBaselineTests.swift

@@ -610,7 +610,6 @@ private final class PerfNostrContext: ChatNostrContext {
     func appendGeohashMessageIfAbsent(_ message: BitchatMessage, toGeohash geohash: String) -> Bool { true }
 
     private(set) var handledPublicMessageCount = 0
-    func handlePublicMessage(_ message: BitchatMessage) { handledPublicMessageCount += 1 }
     func handlePublicMessage(_ message: BitchatMessage, powBits: Int) { handledPublicMessageCount += 1 }
     func checkForMentions(_ message: BitchatMessage) {}
     func sendHapticFeedback(for message: BitchatMessage) {}
@@ -668,8 +667,6 @@ private final class PerfNostrContext: ChatNostrContext {
 
     func favoriteRelationship(forNoiseKey noiseKey: Data) -> FavoritesPersistenceService.FavoriteRelationship? { nil }
     func allFavoriteRelationships() -> [FavoritesPersistenceService.FavoriteRelationship] { [] }
-    func addFavorite(noiseKey: Data, nostrPublicKey: String?, nickname: String) {}
-    func postLocalNotification(title: String, body: String, identifier: String) {}
     func notifyGeohashActivity(geohash: String, bodyPreview: String) {}
 }
 
@@ -779,7 +776,6 @@ private final class PerfDeliveryContext: ChatDeliveryContext {
 @MainActor
 private final class PerfPipelineFixture {
     let viewModel: ChatViewModel
-    let transport: MockTransport
     let conversations: ConversationStore
     let privateInbox: PrivateInboxModel
     let publicChat: PublicChatModel
@@ -791,7 +787,6 @@ private final class PerfPipelineFixture {
         let transport = MockTransport()
         let conversations = ConversationStore()
 
-        self.transport = transport
         self.conversations = conversations
         self.viewModel = ChatViewModel(
             keychain: keychain,