Redesign builder UI: Pico.css + Ableton-style arrangement matrix

Fixes the unreadable white-bar controls and the weak card-list UX.

• Adopt Pico.css (CDN) for accessible, high-contrast chrome; set color-scheme:dark so native selects/range/number controls stop rendering as white bars with light text.

• Replace the vertical section cards with a clip matrix: layers (Pads/Bass/ Drums/Melody/Found) as color-coded rows, sections as time columns scaled by cycle count.

• Transition gutters between columns: ⚡ toggles "fill into next", ＋ inserts a section; per-section name / type / cycles / reorder / delete in the header.

• Color used as meaning (one hue per layer), with a legend.

▶ Play it: Perforated Afterlife — the anchor authored from a measured corpus of real melodies https://stellate.app/?genre=ragtime&seed=9722&path=753.14435,772.14468,734.14468&xdur=13.122

619428c https://github.com/aboard-io/stellate/commit/619428ca30b9b064fa05d5c6c9cf9ba49d3559c2