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>

bitchatTests/ChatComposerCoordinatorContextTests.swift

@@ -82,7 +82,7 @@ struct ChatComposerCoordinatorContextTests {
         context.meshNicknamesByPeerID = [
             PeerID(str: "1111111111111111"): "alice",
             PeerID(str: "2222222222222222"): "bob",
-            PeerID(str: "3333333333333333"): "me",
+            PeerID(str: "3333333333333333"): "me"
         ]
 
         // Matching query: suggestions and range are published, index resets.
@@ -113,7 +113,7 @@ struct ChatComposerCoordinatorContextTests {
             "aaaabbbbccccdddd": "carol",
             // Own token (nickname#last-4-of-pubkey) must be removed; the dummy
             // identity's public key hex ends in "2222".
-            "ffffeeeeddddcccc2222": "me",
+            "ffffeeeeddddcccc2222": "me"
         ]
 
         coordinator.updateAutocomplete(for: "@ca", cursorPosition: 3)