feat: wire ContentView router, macOS 3-column layout, scene lifecycle lock

Completes Phase 4 (Task 17) -- ContentView now routes between UnlockView
and the vault UI (NavigationStack on iOS, 3-column NavigationSplitView on
macOS) instead of the leftover SwiftData template, finally resolving the
long-standing "cannot find type Item" build break. Vault locks on
scene-background (iOS) / resign-active (macOS).

Fixes two more real bugs surfaced now that everything actually links
together: EntryEditViewModel.save's dismiss closure needed @escaping
(used inside Task {}), and UnlockView's UTType(filenameExtension:)
needed `import UniformTypeIdentifiers`.

Also restores MyPass.entitlements, which had regressed to an empty
App Group array with Keychain Sharing missing entirely since the
"wire up AutoFill extension target" commit -- likely clobbered by
Xcode reconciling capabilities at some point without a rebuild in
between to catch it. Verified clean `xcodebuild` on both iOS Simulator
and macOS destinations, and `swift test` still passes all 18 tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WYqycDFsynHH9VnnK7LNSf

MyPass/MyPass.entitlements

@@ -3,6 +3,12 @@
 <plist version="1.0">
 <dict>
 	<key>com.apple.security.application-groups</key>
-	<array/>
+	<array>
+		<string>group.org.antiloop222.mypass</string>
+	</array>
+	<key>keychain-access-groups</key>
+	<array>
+		<string>$(AppIdentifierPrefix)org.antiloop222.mypass</string>
+	</array>
 </dict>
 </plist>