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>

.periphery.yml

@@ -0,0 +1,14 @@
+# Periphery dead-code scan configuration (https://github.com/peripheryapp/periphery)
+#
+# CI runs the macOS scheme only (an iOS scan needs a device destination and
+# doubles the build time). macOS-only scans falsely flag iOS-only code —
+# state restoration, screenshot handlers, background BLE sampling — so those
+# findings live in .periphery.baseline.json rather than being "fixed".
+# When auditing by hand, scan BOTH schemes and intersect:
+#   periphery scan --schemes "bitchat (iOS)" -- -destination 'generic/platform=iOS' ARCHS=arm64
+project: bitchat.xcodeproj
+schemes:
+  - bitchat (macOS)
+retain_swift_ui_previews: true
+relative_results: true
+baseline: .periphery.baseline.json