feat: add settings import/export

[zdennisz]

Summary

• Adds "Backup & Restore" section to Settings > General page
• Export creates a ZIP of ~/homebrew/settings/ and ~/homebrew/data/ with a manifest
• Import restores from a previously exported ZIP, merging into existing dirs
• Uses existing file picker modal for folder/file selection
• Accessible directly from Gaming Mode — no Desktop Mode needed

Closes #904

Implementation

Backend (utilities.py):

• utilities/export_settings — takes destination path, produces decky_backup_<timestamp>.zip
• utilities/import_settings — takes zip path, extracts settings + data

Frontend (settings/pages/general/index.tsx):

• Export button → folder picker → creates ZIP → toast notification
• Import button → file picker (.zip) → restores → toast with count

What's included in export:

• settings/ — loader.json + per-plugin settings
• data/ — per-plugin runtime data (themes, configs)
• manifest.json — version, date, plugin list

What's NOT included:

• plugins/ — reinstallable from store
• logs/ — not useful for backup
• services/ — the binary itself

Test plan

• Export from General Settings → ZIP created with correct structure
• Import ZIP → settings/data restored, toast shows count
• Export on device A, import on device B → cross-device migration
• Import with no existing settings dir → dirs created
• Large data/ dir (CSSLoader themes) → exports without issues

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