Fix search state persisting across tabs

[AusafMo]

Summary

• Clear transient search state when switching app tabs
• Cancel stale text/image search requests so late responses cannot repopulate results
• Guard SwiftUI previews for CLI typechecking outside Xcode

Testing

• Manually tested patched app bundle: search on Searchy, switch to Volumes/Gallery, confirmed search clears
• swiftc -typecheck searchy/*.swift -target arm64-apple-macos14.0 -sdk "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk"
• .venv/bin/ruff check searchy/backend/.py searchy/backend/routes/.py
• make test

Note: local pre-push xcodebuild cannot run because this machine is using Command Line Tools, not full Xcode.

[AusafMo]

lgtm