Add Spotify integration with toggleable music source

- Add OAuth 2.0 PKCE authentication for Spotify Web API
- Create SpotifyNowPlayingMonitor for polling current track
- Add Settings tab with music source toggle (Apple Music/Spotify)
- Store tokens securely in Keychain
- Display current track on Glass as NOW_PLAYING card

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

IrisCompanion/iris/Spotify/SpotifyModels.swift

@@ -0,0 +1,91 @@
+//
+//  SpotifyModels.swift
+//  iris
+//
+//  Codable models for Spotify Web API responses.
+//
+
+import Foundation
+
+struct SpotifyTokens: Codable {
+    let accessToken: String
+    let refreshToken: String
+    let expiresAt: Date
+
+    enum CodingKeys: String, CodingKey {
+        case accessToken = "access_token"
+        case refreshToken = "refresh_token"
+        case expiresAt = "expires_at"
+    }
+
+    var isExpired: Bool {
+        Date() >= expiresAt
+    }
+
+    var expiresWithinMinutes: Bool {
+        Date().addingTimeInterval(5 * 60) >= expiresAt
+    }
+}
+
+struct SpotifyTokenResponse: Codable {
+    let accessToken: String
+    let tokenType: String
+    let expiresIn: Int
+    let refreshToken: String?
+    let scope: String?
+
+    enum CodingKeys: String, CodingKey {
+        case accessToken = "access_token"
+        case tokenType = "token_type"
+        case expiresIn = "expires_in"
+        case refreshToken = "refresh_token"
+        case scope
+    }
+}
+
+struct SpotifyPlaybackState: Codable {
+    let isPlaying: Bool
+    let progressMs: Int?
+    let item: SpotifyTrack?
+
+    enum CodingKeys: String, CodingKey {
+        case isPlaying = "is_playing"
+        case progressMs = "progress_ms"
+        case item
+    }
+}
+
+struct SpotifyTrack: Codable {
+    let id: String
+    let name: String
+    let artists: [SpotifyArtist]
+    let album: SpotifyAlbum
+    let durationMs: Int
+
+    enum CodingKeys: String, CodingKey {
+        case id, name, artists, album
+        case durationMs = "duration_ms"
+    }
+}
+
+struct SpotifyArtist: Codable {
+    let id: String
+    let name: String
+}
+
+struct SpotifyAlbum: Codable {
+    let id: String
+    let name: String
+}
+
+enum MusicSource: String, CaseIterable, Codable {
+    case appleMusic = "apple_music"
+    case spotify = "spotify"
+
+    var displayName: String {
+        switch self {
+        case .appleMusic: return "Apple Music"
+        case .spotify: return "Spotify"
+        }
+    }
+}