fix(settings): persist the "System" theme choice

InterlinedList/Views/SettingsView.swift

@@ -95,9 +95,7 @@ struct SettingsView: View {
                 // Guard against spurious saves when syncFromUser sets the initial value on appear.
                 let serverTheme = authState.user?.theme ?? "system"
                 guard newValue != serverTheme else { return }
-                // "system" means no explicit preference — send nil so the server clears it.
-                // Sending the string "system" is rejected or treated as default (light) by the server.
-                Task { await save(theme: newValue == "system" ? nil : newValue) }
+                Task { await save(theme: newValue) }
             }
         }
     }

InterlinedListTests/APIClientTests/APIClientProfileTests.swift

@@ -159,4 +159,25 @@ final class APIClientProfileTests: XCTestCase {
         let user = try await sut.updateUserSettings(theme: "light")
         XCTAssertEqual(user.username, "alice")
     }
+
+    func test_updateUserSettings_systemTheme_sendsLiteralSystemString() async throws {
+        session.stub(json: #"{"user":\#(userJSON)}"#)
+        _ = try await sut.updateUserSettings(theme: "system")
+        let body = try XCTUnwrap(session.lastRequest?.httpBody)
+        let json = try XCTUnwrap(try? JSONSerialization.jsonObject(with: body) as? [String: Any])
+        XCTAssertEqual(json["theme"] as? String, "system", #"Body must be {"theme":"system"}"#)
+        XCTAssertEqual(json.count, 1, "Only the changed field belongs in the body")
+    }
+
+    func test_updateUserSettings_nilTheme_omitsThemeKey() async throws {
+        // Synthesized Encodable uses encodeIfPresent, and /api/user/update applies
+        // `...(theme !== undefined && { theme })` — so a nil theme is a silent no-op,
+        // never a clear. Callers must send "system" verbatim to select the OS theme.
+        session.stub(json: #"{"user":\#(userJSON)}"#)
+        _ = try await sut.updateUserSettings(theme: nil, showAdvancedPostSettings: true)
+        let body = try XCTUnwrap(session.lastRequest?.httpBody)
+        let json = try XCTUnwrap(try? JSONSerialization.jsonObject(with: body) as? [String: Any])
+        XCTAssertNil(json["theme"])
+        XCTAssertEqual(json.count, 1)
+    }
 }