feat(settings): Preferences section skeleton + Language setting

Marks .preferences implemented, adds the section header/description and
a Display subsection. Language is real and wired end-to-end (users.update
language field, high confidence per Outline's public API docs) — the rest
of the toggles land in follow-up commits, one setting at a time.

Outpost/Features/Account/SettingsView.swift

@@ -32,6 +32,8 @@ struct SettingsView: View {
     @State private var editableName = ""
     @State private var isSavingName = false
     @State private var nameErrorMessage: String?
+    @State private var isSavingLanguage = false
+    @State private var languageErrorMessage: String?
 
     /// Full Local Sync and cache-clearing both need a real connection to be
     /// safe — clearing while offline (or letting Full Local Sync think it
@@ -71,6 +73,7 @@ struct SettingsView: View {
         switch section {
         case .appearance: appearanceDetail
         case .profile: profileDetail
+        case .preferences: preferencesDetail
         case .offlineSync: offlineSyncDetail
         case .advanced: advancedDetail
         case .about: aboutDetail
@@ -231,6 +234,84 @@ struct SettingsView: View {
         .frame(maxWidth: 420, alignment: .leading)
     }
 
+    // MARK: - Preferences
+
+    private var preferencesDetail: some View {
+        VStack(alignment: .leading, spacing: 24) {
+            sectionHeader
+            Text("Manage settings that affect your personal experience.")
+                .font(.subheadline)
+                .foregroundStyle(.secondary)
+
+            preferencesSubsection("Display") {
+                languageRow
+            }
+        }
+        .frame(maxWidth: 480, alignment: .leading)
+        .task { await refreshProfile() }
+    }
+
+    private func preferencesSubsection<Content: View>(
+        _ title: String,
+        @ViewBuilder content: () -> Content
+    ) -> some View {
+        VStack(alignment: .leading, spacing: 14) {
+            Text(title)
+                .font(.headline)
+            content()
+        }
+    }
+
+    private var languageRow: some View {
+        VStack(alignment: .leading, spacing: 6) {
+            HStack {
+                VStack(alignment: .leading, spacing: 2) {
+                    Text("Language")
+                    Text("Choose the interface language. Community translations are accepted through our translation portal.")
+                        .font(.caption)
+                        .foregroundStyle(.secondary)
+                }
+                Spacer()
+                if isSavingLanguage {
+                    ProgressView().controlSize(.small)
+                } else {
+                    Picker("Language", selection: languageBinding) {
+                        ForEach(OutlineLocale.all) { locale in
+                            Text(locale.label).tag(locale.code)
+                        }
+                    }
+                    .labelsHidden()
+                    .frame(width: 200)
+                }
+            }
+            if let languageErrorMessage {
+                Text(languageErrorMessage)
+                    .font(.caption)
+                    .foregroundStyle(.red)
+            }
+        }
+    }
+
+    private var languageBinding: Binding<String> {
+        Binding(
+            get: { session.userLanguage ?? "en_US" },
+            set: { newValue in Task { await saveLanguage(newValue) } }
+        )
+    }
+
+    private func saveLanguage(_ code: String) async {
+        guard code != session.userLanguage, let apiClient = session.apiClient, let userId = session.userId else { return }
+        isSavingLanguage = true
+        defer { isSavingLanguage = false }
+        do {
+            let updated = try await apiClient.updateUserLanguage(UpdateUserLanguageRequest(id: userId, language: code))
+            session.applyUpdatedProfile(updated)
+            languageErrorMessage = nil
+        } catch {
+            languageErrorMessage = outlineErrorMessage(error, fallback: "Couldn't update your language.")
+        }
+    }
+
     // MARK: - Offline & Sync
 
     private var offlineSyncDetail: some View {

Outpost/Root/AppNavigation.swift

@@ -122,7 +122,7 @@ enum SettingsSection: String, CaseIterable, Identifiable, Hashable {
     /// placeholder until its content is specified and built.
     var isImplemented: Bool {
         switch self {
-        case .appearance, .offlineSync, .advanced, .about, .profile:
+        case .appearance, .offlineSync, .advanced, .about, .profile, .preferences:
             return true
         default:
             return false

Outpost/Support/OutlineLocale.swift

@@ -0,0 +1,34 @@
+import Foundation
+
+/// Outline's interface-language options. Not exhaustive — Outline accepts
+/// community translations via its own translation portal, so the real list
+/// on any given server can be longer than this; this covers the common
+/// cases and falls back to showing whatever code the server already has
+/// set even if it isn't in this list.
+struct OutlineLocale: Identifiable, Hashable {
+    let code: String
+    let label: String
+
+    var id: String { code }
+
+    static let all: [OutlineLocale] = [
+        OutlineLocale(code: "en_US", label: "English (US)"),
+        OutlineLocale(code: "de_DE", label: "Deutsch"),
+        OutlineLocale(code: "fr_FR", label: "Français"),
+        OutlineLocale(code: "es_ES", label: "Español"),
+        OutlineLocale(code: "pt_PT", label: "Português"),
+        OutlineLocale(code: "pt_BR", label: "Português (Brasil)"),
+        OutlineLocale(code: "it_IT", label: "Italiano"),
+        OutlineLocale(code: "nl_NL", label: "Nederlands"),
+        OutlineLocale(code: "pl_PL", label: "Polski"),
+        OutlineLocale(code: "ru_RU", label: "Русский"),
+        OutlineLocale(code: "ja_JP", label: "日本語"),
+        OutlineLocale(code: "ko_KR", label: "한국어"),
+        OutlineLocale(code: "zh_CN", label: "中文 (简体)"),
+        OutlineLocale(code: "zh_TW", label: "中文 (繁體)"),
+    ]
+
+    static func label(for code: String) -> String {
+        all.first(where: { $0.code == code })?.label ?? code
+    }
+}