Mobile Resize v3 — per-section "stack on mobile" toggle

The rule, refined:

• Default behaviour by section type:
  • Card / Gallery / Icon list / Team grid / Image pair → keep 2-col on mobile (looks better with short text)
  • Text / Heading / Hero / Long-form → stack on mobile (reading needs full width)
• Per-section override: a small toggle on every section (2-col · stack) lets the operator flip the default. Lives in the section's quick settings and as a small badge on hover.
• Column ratio: when keeping 2-col, ratio is independent of desktop — 50/50 by default, 60/40, 33/66 etc. as buttons (not drag, on a 390px screen precision-drag is hellish).
• Auto-collapse breakpoint: the operator sets the threshold (default 480px). Below it, even 2-col sections stack. Lets the operator say "this 2-col should be 2-col on iPad mini but stack on iPhone SE".
• Gap control: per-section. Default 8px. Slider on the mobile settings panel.
• Live preview: bottom of the mobile settings panel shows the actual layout at the current viewport width. No guessing.

The insight: don't make a global "stack on mobile" decision. Some sections want 2-col (cards), some want stack (text). Make it a per-section property with a sensible default by type.