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/Services/NostrRelayManagerTests.swift

@@ -1483,7 +1483,6 @@ final class NostrRelayManagerTests: XCTestCase {
         return RelayManagerTestContext(
             manager: manager,
             permissionSubject: permissionSubject,
-            favoritesSubject: favoritesSubject,
             sessionFactory: sessionFactory,
             scheduler: scheduler,
             clock: clock,
@@ -1537,7 +1536,6 @@ final class NostrRelayManagerTests: XCTestCase {
 private struct RelayManagerTestContext {
     let manager: NostrRelayManager
     let permissionSubject: CurrentValueSubject<LocationChannelManager.PermissionState, Never>
-    let favoritesSubject: CurrentValueSubject<Set<Data>, Never>
     let sessionFactory: MockRelaySessionFactory
     let scheduler: MockRelayScheduler
     let clock: MutableClock
@@ -1644,7 +1642,6 @@ private final class MockRelaySessionFactory: NostrRelaySessionProtocol {
 }
 
 private final class MockRelayConnection: NostrRelayConnectionProtocol {
-    private let url: String
     private let pingError: Error?
     private let sendError: Error?
     private var receiveHandler: ((Result<URLSessionWebSocketTask.Message, Error>) -> Void)?
@@ -1662,8 +1659,7 @@ private final class MockRelayConnection: NostrRelayConnectionProtocol {
         }
     }
 
-    init(url: String, pingError: Error? = nil, sendError: Error? = nil) {
-        self.url = url
+    init(url _: String, pingError: Error? = nil, sendError: Error? = nil) {
         self.pingError = pingError
         self.sendError = sendError
     }