Hook up Damus Purple translation service

This commit integrates the Damus Purple translation service:
- Automatically handles translation settings change after purchase
- Asks for permission to override translation settings if the user already has translation setup
- Translation settings can be changed with Damus Purple, if desired
- Translation requests working with the Damus API server

Testing
--------

PASS

Device: iPhone 15 simulator
iOS: 17.2
Damus: This commit
Damus Purple API server: `9397201d7d55ddcec4c18fcd337f759b61dce697` running on Ubuntu 22.04 LTS VM (npm run dev)
iOS setting: English set as the only preferred language.
Steps:
1. Enable Damus Purple feature flag on developer settings, set purple localhost mode, and restart app
2. Set translation setting to something other than none (e.g. DeepL)
3. Simulate Damus Purple purchase
4. Check that when dismissing welcome view, a confirmation prompt will ask the user whether they want to switch translator to Damus Purple. PASS
5. Click "Yes".
6. Go to translation settings. Check that translation settings are set to "Purple". PASS
7. Go to a non-English profile. Check that translations appear with "Mock translation" (Which is the translation text provided by the mock translation server). PASS
8. Reinstall app
9. Repeat the test, but this time starting with no translation settings. Make sure that translation settings will automatically switch to Damus Purple. PASS

Feature flag testing
--------------------

PASS

Preconditions: Same as above
Steps:
1. Turn off translation
2. Turn off Damus Purple feature flag
3. Go to translation settings. Make sure that Damus Purple is not an option. PASS

Closes: https://github.com/damus-io/damus/issues/1836
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Models/Purple/DamusPurple.swift

@@ -126,6 +126,36 @@ class DamusPurple: StoreObserverDelegate {
             }
         }
     }
+    
+    func translate(text: String, source source_language: String, target target_language: String) async throws -> String {
+        var url = environment.get_base_url()
+        url.append(path: "/translate")
+        url.append(queryItems: [
+            .init(name: "source", value: source_language),
+            .init(name: "target", value: target_language),
+            .init(name: "q", value: text)
+        ])
+        let (data, response) = try await make_nip98_authenticated_request(
+            method: .get,
+            url: url,
+            payload: nil,
+            payload_type: nil,
+            auth_keypair: self.keypair
+        )
+        
+        if let httpResponse = response as? HTTPURLResponse {
+            switch httpResponse.statusCode {
+                case 200:
+                    return try JSONDecoder().decode(TranslationResult.self, from: data).text
+                default:
+                    Log.error("Translation error with Damus Purple. HTTP status code: %d; Response: %s", for: .damus_purple, httpResponse.statusCode, String(data: data, encoding: .utf8) ?? "Unknown")
+                    throw PurpleError.translation_error(status_code: httpResponse.statusCode, response: data)
+            }
+        }
+        else {
+            throw PurpleError.translation_no_response
+        }
+    }
 }
 
 // MARK: API types
@@ -155,4 +185,13 @@ extension DamusPurple {
             }
         }
     }
+    
+    enum PurpleError: Error {
+        case translation_error(status_code: Int, response: Data)
+        case translation_no_response
+    }
+    
+    struct TranslationResult: Codable {
+        let text: String
+    }
 }

damus/Models/TranslationService.swift

@@ -29,6 +29,7 @@ enum TranslationService: String, CaseIterable, Identifiable, StringCodable {
     }
 
     case none
+    case purple
     case libretranslate
     case deepl
     case nokyctranslate
@@ -38,6 +39,8 @@ enum TranslationService: String, CaseIterable, Identifiable, StringCodable {
         switch self {
         case .none:
             return .init(tag: self.rawValue, displayName: NSLocalizedString("none_translation_service", value: "None", comment: "Dropdown option for selecting no translation service."))
+        case .purple:
+            return .init(tag: self.rawValue, displayName: NSLocalizedString("Damus Purple", comment: "Dropdown option for selecting Damus Purple as a translation service."))
         case .libretranslate:
             return .init(tag: self.rawValue, displayName: NSLocalizedString("LibreTranslate (Open Source)", comment: "Dropdown option for selecting LibreTranslate as the translation service."))
         case .deepl:

damus/Models/UserSettingsStore.swift

@@ -296,6 +296,8 @@ class UserSettingsStore: ObservableObject {
         switch translation_service {
         case .none:
             return false
+        case .purple:
+            return true
         case .libretranslate:
             return URLComponents(string: libretranslate_url) != nil
         case .deepl: