Burn down SwiftLint advisory violations from 109 to 4 (#1362)

Mechanical style fixes across the enabled rule set, mostly via
swiftlint --fix (trailing_comma, comma, colon, trailing_newline,
comment_spacing, unused_closure_parameter, unneeded_break_in_switch,
opening_brace) plus hand fixes:

- non_optional_string_data_conversion (45): .data(using: .utf8)! and
  ?? Data() fallbacks replaced with the non-optional Data(_.utf8),
  including two production sites (NIP-44 HKDF info constant and the
  announce canonicalization context/nickname bytes — byte-identical
  output, only the impossible-nil handling is gone).
- switch_case_alignment: LocationChannel had a misindented closing
  brace; also repaired an --fix artifact in BLEService's .none case.
- redundant_string_enum_value: TrustLevel raw values equal to the case
  names (encoded form unchanged).
- unused_optional_binding: let _ = binds replaced with != nil / is Bool.
- static_over_final_class: PreviewView.layerClass.
- Resolved the BinaryProtocolTests TODO by documenting that 8-byte
  recipient ID truncation is the fixed wire-field size, not a bug.

The 4 remaining violations are all todo markers for a shared
test-helpers module (tracked in #1088) and one Reuse note.

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

bitchat/ViewModels/ChatNostrCoordinator.swift

@@ -71,7 +71,7 @@ final class ChatNostrCoordinator {
         key: Data?
     ) {
         guard let context else { return }
-        if let _ = key {
+        if key != nil {
             if let identity = context.currentNostrIdentity() {
                 context.sendGeohashDeliveryAck(for: message.id, toRecipientHex: senderPubkey, from: identity)
             }
@@ -84,7 +84,7 @@ final class ChatNostrCoordinator {
         }
 
         if !wasReadBefore && context.selectedPrivateChatPeer == message.senderPeerID {
-            if let _ = key {
+            if key != nil {
                 if let identity = context.currentNostrIdentity() {
                     context.sendGeohashReadReceipt(message.id, toRecipientHex: senderPubkey, from: identity)
                 }