feat: prompt for Full Disk Access on the welcome screen

[petercrocker]

Summary

Without Full Disk Access, macOS silently hides folders such as Mail, Messages and other apps' data, so scans under-report sizes with no explanation. The welcome screen now detects this and walks the user to the right setting.

Depends on #4. Review only the last commit until that merges.

Key Changes

• Detects missing Full Disk Access by probing a TCC-protected file (no prompt or side effects)
• Dismissible banner on the welcome screen with Open System Settings (deep-links to the Full Disk Access pane), Relaunch (only when running as an app bundle) and Not Now
• Re-checks when the app becomes active, so the banner clears once access is granted

Documentation Updates

• README: new Full Disk Access section covering what it unlocks, how to grant it, and that ad-hoc signed rebuilds reset the permission

Test Plan

• Install via scripts/bundle-app.sh --install with Full Disk Access off: banner appears
• Open System Settings → enable MacDirStat → switch back / Relaunch: banner disappears
• swift run from a terminal that has Full Disk Access: no banner (the process inherits the terminal's access)
• Tested on the macOS 27 beta only

🤖 Generated with Claude Code

[phalladar]

@petercrocker Thanks for this! Without Full Disk Access, macOS hides folders and scans come up short with no explanation, which users can't figure out on their own. This banner fixes that.

You'd allowed maintainer edits, so I rebased this onto main myself to get it ready to merge. What changed:

• Dropped the feat: add app bundle script and app icon #4 commit. main now has scripts/build-app.sh (feat: add script to package MacDirStat.app #7) for building the .app and a new app icon (Replace app icon with the treemap tiles design #9), so feat: add app bundle script and app icon #4 is superseded and I'm closing it. The clone-directory casing fix is already on main too.
• Dropped the README changes, and removed the line about them from the commit message. They conflicted with main, and the README is being reworked right now. We'll mention the banner in the new version.
• Your code is unchanged. It's the same two files, and you're still the commit author.

Before pushing, I checked that it builds under Swift 6 strict concurrency. I also confirmed the probe behaves as intended without Full Disk Access: the protected files are visible to stat, but open fails with "Operation not permitted", so the banner shows.