The studio's home and app pages lay main and rail with `.split`, and `.dash` leaves the stylesheet #4748

Intent: one two-column layout (main plus a rail) instead of two that differ only by the rail's width.

Done:

• [ ] src/web/studio/home.ts and src/web/studio/app.ts draw <div class="split"> where they draw .dash today; the app page's 320 px rail is kept by a data-rail="wide" attribute on .split, documented in §5d
• [ ] .dash leaves src/web/style/content.ts (its rule and the shared min-width: 0 selector) and src/web/style/components.ts (the :is(.stack, .dash, .split, .timeline) selector), and its name leaves docs/DESIGN.md §5d
• [ ] the rail's stickiness is decided on purpose: .split > aside is sticky; say in the pull whether the studio's rail takes it
• [ ] a test asserts neither page's markup carries dash; a look at 375 px and 1280 px shows no layout shift against main

Risk: dash-main and dash-rail have no rules; rename or drop them in the same pull.

Run bun scripts/design-lint.ts and bun test test/design-lint.test.ts before pushing; where counts fall, lower docs/design/baseline.json in the same pull. Keep the diff to the files named.