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>

bitchat/Models/BitchatMessage+Media.swift

@@ -13,13 +13,6 @@ extension BitchatMessage {
     enum Media {
         case voice(URL)
         case image(URL)
-
-        var url: URL {
-            switch self {
-            case .voice(let url), .image(let url):
-                return url
-            }
-        }
     }
 
     // Cache the directory lookup to avoid repeated FileManager calls during view rendering

bitchat/Models/BitchatPeer.swift

@@ -7,7 +7,6 @@ struct BitchatPeer: Equatable {
     let peerID: PeerID // Hex-encoded peer ID
     let noisePublicKey: Data
     let nickname: String
-    let lastSeen: Date
     let isConnected: Bool
     let isReachable: Bool
     
@@ -77,14 +76,13 @@ struct BitchatPeer: Equatable {
         peerID: PeerID,
         noisePublicKey: Data,
         nickname: String,
-        lastSeen: Date = Date(),
+        lastSeen _: Date = Date(),
         isConnected: Bool = false,
         isReachable: Bool = false
     ) {
         self.peerID = peerID
         self.noisePublicKey = noisePublicKey
         self.nickname = nickname
-        self.lastSeen = lastSeen
         self.isConnected = isConnected
         self.isReachable = isReachable