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/IntentSnippetViews.swift

@@ -0,0 +1,53 @@
+import SwiftUI
+
+/// Compact card Siri shows after saving / opening a bookmark.
+struct BookmarkSnippetView: View {
+    let entity: BookmarkEntity
+
+    var body: some View {
+        HStack(spacing: 12) {
+            Image(systemName: "bookmark.fill")
+                .font(.title2)
+                .foregroundStyle(.tint)
+            VStack(alignment: .leading, spacing: 3) {
+                Text(entity.title)
+                    .font(.headline)
+                    .lineLimit(2)
+                Text(entity.host)
+                    .font(.subheadline)
+                    .foregroundStyle(.secondary)
+                if !entity.tags.isEmpty {
+                    Text(entity.tags.map { "#\($0)" }.joined(separator: " "))
+                        .font(.caption)
+                        .foregroundStyle(.secondary)
+                        .lineLimit(1)
+                }
+            }
+            Spacer(minLength: 0)
+        }
+        .padding()
+    }
+}
+
+/// Card Siri shows for an AI summary result.
+struct SummarySnippetView: View {
+    let title: String
+    let host: String
+    let summary: String
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 8) {
+            HStack(spacing: 8) {
+                Image(systemName: "sparkles").foregroundStyle(.tint)
+                Text(title).font(.headline).lineLimit(2)
+            }
+            Text(host)
+                .font(.caption)
+                .foregroundStyle(.secondary)
+            Text(summary)
+                .font(.body)
+                .fixedSize(horizontal: false, vertical: true)
+        }
+        .padding()
+    }
+}