v3.13.0 — Help Overhaul, 4-Tier Quality, Compare Tool, Multi-Folder

What's New

• Two-tab Help — Usage (shortcuts & features) + Background (comprehensive FAQ covering quality scoring, metrics, sorting, STF stretch, debayer, denoise, deconvolution, triage tips)
• 4-Tier Quality Icons — Full green (excellent), half-green (good), orange (borderline), red (garbage). Z-score on hover
• Compare with Best (C key) — Side-by-side synchronized zoom/pan at 300% zoom. ESC to close
• Metric Bar Indicators — Per-group red-to-green bars below Stars, FWHM, HFR, SNR
• Multi-Folder Opening — Cmd+click multiple folders in open dialog → merged session
• Context Menu — Open With... (PixInsight etc.), Show in Finder, Compare with Best
• Quality Group Size — Minimum reduced from 20 to 10 images per group
• Pitch-Black Detection — Frames with no data auto-flagged as garbage
• Smart 4-Case Sorting — Auto-sort by session type with exposure as grouping element
• FITS Special Characters — Brackets/parentheses in filenames now work correctly
• Splash "Don't show on startup" checkbox

Bug Fixes

• Initial sort fires correctly after first precache
• Metric bars use proportional constraints (no grow-on-select)
• FWHM/HFR sort ascending by default (lower = better)
• PNG export R/B channel swap fix