fix: security hardening — Keychain, no hardcoded creds, safe URLs

- Add KeychainService for encrypted token storage (auth, refresh, health JWT, API key)
- Remove hardcoded email/password from HealthAPIService, store in Keychain
- Move all tokens from UserDefaults to Keychain
- API key sent via X-API-Key header instead of URL query parameter
- Replace force unwrap URL(string:)! with guard let + throws
- Fix force unwrap Calendar.date() in HealthKitService
- Mark HealthKitService @MainActor for thread-safe @Published
- Use withTaskGroup for parallel habit log fetching in TrackerView
- Check notification permission before scheduling reminders
- Add input validation (title max 200 chars)
- Add privacy policy and terms links in Settings
- Update CLAUDE.md with security section

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PulseHealth/Services/KeychainService.swift

@@ -0,0 +1,49 @@
+import Foundation
+import Security
+
+enum KeychainService {
+    static let service = "com.daniil.pulsehealth"
+
+    static func save(key: String, value: String) {
+        guard let data = value.data(using: .utf8) else { return }
+        let query: [String: Any] = [
+            kSecClass as String: kSecClassGenericPassword,
+            kSecAttrService as String: service,
+            kSecAttrAccount as String: key
+        ]
+        SecItemDelete(query as CFDictionary)
+        var add = query
+        add[kSecValueData as String] = data
+        add[kSecAttrAccessible as String] = kSecAttrAccessibleAfterFirstUnlock
+        SecItemAdd(add as CFDictionary, nil)
+    }
+
+    static func load(key: String) -> String? {
+        let query: [String: Any] = [
+            kSecClass as String: kSecClassGenericPassword,
+            kSecAttrService as String: service,
+            kSecAttrAccount as String: key,
+            kSecReturnData as String: true,
+            kSecMatchLimit as String: kSecMatchLimitOne
+        ]
+        var result: AnyObject?
+        guard SecItemCopyMatching(query as CFDictionary, &result) == errSecSuccess,
+              let data = result as? Data else { return nil }
+        return String(data: data, encoding: .utf8)
+    }
+
+    static func delete(key: String) {
+        let query: [String: Any] = [
+            kSecClass as String: kSecClassGenericPassword,
+            kSecAttrService as String: service,
+            kSecAttrAccount as String: key
+        ]
+        SecItemDelete(query as CFDictionary)
+    }
+
+    // Keys
+    static let tokenKey = "auth_token"
+    static let refreshTokenKey = "auth_refresh_token"
+    static let healthTokenKey = "health_jwt_token"
+    static let healthApiKeyKey = "health_api_key"
+}