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.xcodeproj/xcshareddata/xcschemes/ShareExtension.xcscheme

@@ -1,10 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
    LastUpgradeVersion = "1630"
-   version = "1.3">
+   version = "1.7">
    <BuildAction
       parallelizeBuildables = "YES"
-      buildImplicitDependencies = "YES">
+      buildImplicitDependencies = "YES"
+      runPostActionsOnFailure = "NO">
       <BuildActionEntries>
          <BuildActionEntry
             buildForTesting = "YES"
@@ -40,7 +41,8 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
-      shouldUseLaunchSchemeArgsEnv = "YES">
+      shouldUseLaunchSchemeArgsEnv = "YES"
+      onlyGenerateCoverageForSpecifiedTargets = "NO">
       <MacroExpansion>
          <BuildableReference
             BuildableIdentifier = "primary"
@@ -65,15 +67,6 @@
       allowLocationSimulation = "YES">
       <BuildableProductRunnable
          runnableDebuggingMode = "0">
-         <BuildableReference
-            BuildableIdentifier = "primary"
-            BlueprintIdentifier = "CF31B2655F8F30CF6DF3C26F"
-            BuildableName = "ShareExtension.appex"
-            BlueprintName = "ShareExtension"
-            ReferencedContainer = "container:Marks.xcodeproj">
-         </BuildableReference>
-      </BuildableProductRunnable>
-      <MacroExpansion>
          <BuildableReference
             BuildableIdentifier = "primary"
             BlueprintIdentifier = "76C272776D8EBCCD3FB6715A"
@@ -81,7 +74,9 @@
             BlueprintName = "Marks"
             ReferencedContainer = "container:Marks.xcodeproj">
          </BuildableReference>
-      </MacroExpansion>
+      </BuildableProductRunnable>
+      <CommandLineArguments>
+      </CommandLineArguments>
    </LaunchAction>
    <ProfileAction
       buildConfiguration = "Release"