App layer: duplicated key handling and Quick Look logic; settings folder lists grow unbounded #46

Findings from the review of Sources/DigUpApp, plus requested UX changes:

• The panel and the search window each carried near-identical key handlers (Esc/arrows/↩/Space/⌘Y/⌘C, input-method and modifier-mask normalization) and verbatim-duplicated Quick Look toggle/update/data-source code — already drifting apart
• SearchWindow.show(query:kind:) set filter then query, firing two search runs
• AppController.setExcluded saved raw paths while applyFolders normalized through FolderSelection — two save paths, different results
• Onboarding.init ran filesystem scans synchronously on the main thread
• The Folders to search and Code folders lists grew the settings page without bound

Fixed by #39 (shared KeyCommands / QuickLookController, single-run show, normalized setExcluded, cached Onboarding scans, four-row scrollable folder lists).