feat(backup): timestamp the exported config filename

[BlackHole1]

Config export always suggested the same "LockIME Backup.lockime" name, so successive exports were indistinguishable in Finder. The suggested name now carries a local-time stamp (e.g. LockIME Backup 2026-06-22 23-15-28.lockime), which is also sortable by name.

The stamp is built from calendar components in a fixed, locale- independent layout rather than a DateFormatter string, so it never leaks the system locale into a name the app's language override is meant to govern.

Feed NSSavePanel the extension-less stem and let allowedContentTypes append the single .lockime: embedding the extension makes the panel append a second one (.lockime.lockime), especially once the stem contains dots. Time fields use "-" (not ":" or ".") so the stem stays filename-legal and free of anything the panel could mistake for an existing extension.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 20e3d521-ed4a-4ea4-953e-1c10619ec670

📥 Commits

Reviewing files that changed from the base of the PR and between 493eb7a and 8fa2000.

📒 Files selected for processing (3)

• Sources/LockIME/UI/Settings/BackupSettingsPane.swift
• Sources/LockIMEKit/Backup/ConfigBackup.swift
• Tests/LockIMEKitTests/ConfigBackupTests.swift

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Summary by CodeRabbit

• New Features

  • Backup export filenames now automatically include date/time stamps, making it easier to identify and organize multiple backups.

• Tests

  • Added comprehensive test coverage for backup filename generation logic.

Walkthrough

ConfigBackup gains three new static members: fileNamePrefix (a fixed ASCII brand string), suggestedFileNameStem(date:timeZone:) (builds a locale-independent yyyy-MM-dd HH-mm-ss stem using Gregorian calendar components in the supplied time zone), and suggestedFileName(date:timeZone:) (appends fileExtension to the stem). BackupSettingsPane.exportConfiguration() replaces the hardcoded "LockIME Backup.<ext>" string with ConfigBackup.suggestedFileNameStem(date: Date()) as the save panel's nameFieldStringValue, while the .lockime extension is still enforced through allowedContentTypes. A new test suite validates formatting, zero-padding, time-zone rendering, character legality, single-dot enforcement, default time-zone behavior, and chronological lexicographic ordering.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'feat(backup): timestamp the exported config filename' follows the required format of type(scope): subject and clearly describes the main change of adding timestamps to exported config filenames.
Description check	✅ Passed	The description comprehensively explains the rationale, implementation approach, and specific technical details (locale-independence, extension handling, timestamp formatting) that directly relate to the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch imporve-export-name

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