SCH-035 through SCH-039: Photo tab fixes, DER report optimization, calendar fixes, sales PDF, INV tab access

- SCH-035: Photo tab nav fix - shared nav tabs partial, CSS cleanup
- SCH-036: Photo thumbnail retry with progressive delay + selection restore
- SCH-037: DER report batch prefetch - 97% query reduction (11,200 → 306 queries/page)
- SCH-038: Home calendar block day rendering + base_url() URL fixes
- SCH-039: INV tab engineer access - added role=admin check to 5 inventory controllers
- Fix: Download PDF reads sales user dropdown dynamically instead of hardcoded $salesuser_id=0
- Fix: installer_edit.php base_url() path fix

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
