Add clipboard history search

[LeoN0425]

Summary

• Add a Search History action to the status menu and a dedicated SwiftUI search window.
• Match case-insensitive contiguous fragments across clipboard text and OCR content, using the existing FTS5 index for longer queries and direct substring matching for short queries.
• Show recent matches with image thumbnails and allow pasting by pressing Return, clicking Paste, or double-clicking a result.
• Restore focus to the previously active application before pasting.
• Localize the new interface and add repository tests for contiguous matching, case insensitivity, quoted and short queries, Chinese text, OCR results, ordering, limits, and thumbnails.

Testing

• PasteboardHistoryRepositoryTests: 15 passed, 0 failed with Xcode 26.6 in an arm64 Release verification build.
• The Release app bundle compiled, linked, and passed ad-hoc code-sign verification.
• A fresh test run after rebasing was blocked during Swift Package resolution by TLS/timeouts while downloading Firebase binary targets from dl.google.com; no compilation or test failure was reported.

[won-seoop]

I opened a focused follow-up against this PR branch: LeoN0425#1

It makes the default Command-Shift-V flow open the search window directly and shows recent clipboard entries before typing, so the panel works as a searchable replacement for the existing clipboard picker. The follow-up stays on top of this PR to preserve the original implementation and authorship.

Verification is complete: the full macOS 26 test suite and arm64 Release build passed, and the packaged app was launched locally to confirm recent-history display and live filtering.