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/Services/NoiseEncryptionService.swift

@@ -428,7 +428,7 @@ final class NoiseEncryptionService {
     private func canonicalAnnounceBytes(peerID: Data, noiseKey: Data, ed25519Key: Data, nickname: String, timestampMs: UInt64) -> Data {
         var out = Data()
         // context
-        let context = "bitchat-announce-v1".data(using: .utf8) ?? Data()
+        let context = Data("bitchat-announce-v1".utf8)
         out.append(UInt8(min(context.count, 255)))
         out.append(context.prefix(255))
         // peerID (expect 8 bytes; pad/truncate to 8 for canonicalization)
@@ -444,7 +444,7 @@ final class NoiseEncryptionService {
         out.append(ed32)
         if ed32.count < 32 { out.append(Data(repeating: 0, count: 32 - ed32.count)) }
         // nickname length + bytes
-        let nickData = nickname.data(using: .utf8) ?? Data()
+        let nickData = Data(nickname.utf8)
         out.append(UInt8(min(nickData.count, 255)))
         out.append(nickData.prefix(255))
         // timestamp