feat: canvas background selector — grid, dot, or blank preference
Add user-selectable canvas background style via data-canvas-bg attribute
and CSS custom properties (--rs-canvas-bg-image, --rs-canvas-bg-size).
Three options: grid (default), dot, blank — persisted in localStorage.
- theme.css: new tokens + [data-canvas-bg] selectors
- rstack-identity.ts: Grid/Dot/Blank selector in user dropdown
- canvas.html: CSS vars, zoom-aware scaling, canvas-bg-change listener
- flows.css: use shared bg-image/bg-size vars (fixes rFlows theme bug)
- FOUC prevention in all entry points (shell.ts, create-space.html)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Jeff Emmett
8072b250ea