Remove obsolete dashboard layout code

[BorisTyshkevich]

What changed

• Removed the disconnected dashLayout/dashCols preference and its legacy Dashboard helpers.
• Removed unused onRaw, the ignored fourth buildCardModel parameter, and the unused Spec-schema import.
• Removed the confirmed dead CSS rules and restored the mobile editor/results splitter selector by adding its class.
• Replaced the dashboard mobile fixture's fake preference persistence with layout-only checks.

Why

The legacy layout system was superseded by the workspace-backed Dashboard layout model. The remaining surface could no longer affect production behavior and left stale state, CSS, and API contracts behind. The mobile CSS targeted a class the splitter did not carry.

Impact

No runtime Dashboard behavior changes. Old asb:dashLayout/asb:dashCols localStorage values are simply ignored. In single-pane mobile editor view, the splitter no longer leaves a visible resize row.

The PNG finding was not actioned: docs/assets/img is referenced by live project documentation, and docs/ui-snapshots is the documented versioned design-review archive.

Validation

• npm test — 6,079 tests passed; coverage gate met (100% statements/lines).
• npx playwright test tests/e2e/tile-open-workbench.spec.js — 34 passed, 2 skipped (browser-platform skips); includes the mobile splitter regression check.
• npm run build — succeeded.