feat(intents): add App Intents + Siri integration

Expose Marks to Siri, Spotlight, and Shortcuts via App Intents:

- BookmarkEntity (AppEntity + IndexedEntity, keyed on the Linkding
  server id) with a server-backed EntityStringQuery for live lookup
- Intents: Add, Open (OpenIntent), Search, ShowUnread, and an AI
  Summarize intent backed by ClaudeService; AppShortcutsProvider with
  spoken phrases; Siri snippet views and dialog
- SearchMarksIntent conforms to the system.search App Schema
  (@AppIntent(schema:), ShowInAppSearchResultsIntent) so it is
  executable by conversational Siri / Apple Intelligence
- IntentRouter bridges intents (run in the app process) to the SwiftUI
  scene for open/search navigation; widgets refreshed after adds
- Link AppIntents.framework so the metadata processor extracts shortcut
  phrases and validates the assistant schema

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Marks/Intents/IntentSupport.swift

@@ -0,0 +1,66 @@
+import Foundation
+import AppIntents
+
+/// Which top-level tab the app is showing. Used so an intent can switch tabs.
+enum AppTab: Hashable {
+    case bookmarks, tags, search
+}
+
+/// Bridges App Intents (which run in the main app process, since there is no
+/// separate AppIntents extension target) to the live SwiftUI scene.
+///
+/// Intents mutate this shared, observable singleton; `MainContainer` and
+/// `SearchView` observe it and react (present a browser, switch to the search
+/// tab, etc.). This is the standard "intent → in-app navigation" pattern when
+/// perform() runs in-process.
+@MainActor
+@Observable
+final class IntentRouter {
+    static let shared = IntentRouter()
+    private init() {}
+
+    /// A bookmark URL an intent asked to open in the in-app browser.
+    var openBookmarkURL: String?
+
+    /// A query an intent asked the app to search for.
+    var searchRequest: String?
+
+    func openBookmark(url: String) { openBookmarkURL = url }
+    func search(_ query: String) { searchRequest = query }
+}
+
+/// Errors surfaced to Siri / Shortcuts when an intent can't run.
+enum MarksIntentError: Error, CustomLocalizedStringResourceConvertible {
+    case notConfigured
+    case invalidURL
+
+    var localizedStringResource: LocalizedStringResource {
+        switch self {
+        case .notConfigured: "Open Marks and connect to your Linkding server first."
+        case .invalidURL: "That doesn't look like a valid link."
+        }
+    }
+}
+
+/// Shared plumbing for the intents: build an API client from saved config and
+/// keep the widgets in sync after mutations.
+enum MarksIntent {
+    /// Builds a `LinkdingAPI` from the saved server config, or throws a
+    /// user-facing error if the app has never been connected.
+    static func api() throws -> LinkdingAPI {
+        guard let config = ServerConfig.load() else { throw MarksIntentError.notConfigured }
+        return LinkdingAPI(config: config)
+    }
+
+    /// Prepend a freshly-saved bookmark into the widget cache so the Recent
+    /// widget reflects it immediately (mirrors `BookmarksViewModel.load`).
+    static func refreshWidgets(adding b: Bookmark) {
+        var items = WidgetDataStore.loadBookmarks()
+        items.removeAll { $0.url == b.url }
+        items.insert(
+            WidgetBookmark(url: b.url, title: b.displayTitle, domain: b.domain, faviconUrl: b.faviconUrl),
+            at: 0
+        )
+        WidgetDataStore.saveBookmarks(Array(items.prefix(20)))
+    }
+}