refactor(setup): improve htaccess update handling and code clarity

[joshtrichards]

• Resolves: #

Summary

Optimize the .htaccess and data-dir protection update handling and make more defensive.

• idempotent now: only update main .htaccess file when the generated block actually changes
• centralized probe bypass logic in isHtaccessWorking
• made read/write/existence checks/error detection a bit more robust/thorough
• low disk space guard now covers entire file body
• additional clarifying docblocks/comments

TODO

• [ ] ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)
• [ ] Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• [ ] Milestone added for target branch/version (ex: 32.x for stable32)