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/LocalizationCoverageTests.swift

@@ -10,7 +10,6 @@ struct LocalizationCoverageTests {
         .deletingLastPathComponent()  // repo root
 
     private struct Catalog {
-        let sourceLanguage: String
         /// key -> set of locales with a localization entry
         let coverage: [String: Set<String>]
         /// all locales appearing anywhere in the catalog
@@ -22,7 +21,6 @@ struct LocalizationCoverageTests {
         let data = try Data(contentsOf: url)
         let root = try #require(try JSONSerialization.jsonObject(with: data) as? [String: Any])
         let strings = try #require(root["strings"] as? [String: Any])
-        let sourceLanguage = try #require(root["sourceLanguage"] as? String)
 
         var coverage: [String: Set<String>] = [:]
         for (key, value) in strings {
@@ -43,7 +41,7 @@ struct LocalizationCoverageTests {
             }
             coverage[key] = locales
         }
-        return Catalog(sourceLanguage: sourceLanguage, coverage: coverage)
+        return Catalog(coverage: coverage)
     }
 
     @Test func mainCatalogCoversAllLocalesForEveryKey() throws {