Validate recent database paths before display and auto-cleanup stale entries

[Copilot]

The recent databases list stored file paths in localStorage without validation. Users saw stale entries for moved/deleted files, and could attempt to open invalid databases.

Changes

Backend: New validate_database_file command

• Checks file existence, type, and .kdbx extension
• Validates KDBX magic bytes (0x03D9A29A) by reading only first 8 bytes
• Returns boolean without exposing sensitive data

Frontend: New getValidatedRecentDatabases() function

• Validates all paths concurrently on mount
• Auto-removes invalid entries from localStorage
• Handles errors gracefully (permissions, I/O failures)

UI: CustomTitleBar now uses validated paths

• "Open Recent" menu shows only accessible databases
• No user-facing changes beyond removing stale entries

// Before: Direct localStorage read, no validation
const recent = getRecentDatabases(); // May include missing/invalid files

// After: Validated and cleaned
const recent = await getValidatedRecentDatabases(); // Only valid KDBX files

Concurrent validation is bounded to 10 items (enforced by addRecentDatabase). Tauri provides built-in timeout protection for invoke calls.

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