feat(editor): light mode

[NiJingzhe]

What

Light mode for sca edit.

• Follows the operating system's theme; a button at the end of the header cycles system → light → dark and the choice is remembered (localStorage["sca-theme"]). A small script in index.html sets the theme before first paint, so there is no flash.
• The page palette is two sets of CSS variables (light is the default, html.dark restores the existing dark look); the Tailwind black/white literals (about 25 sites) became tokens. Light keeps the square black-and-white look with the four meaning inks darkened for contrast.
• The 3D viewport goes light too (src/scene/theme.ts): background, edge color, grid, axis gizmo, vertex points and the selection highlight. BRepRenderer.setTheme recolors in place, so the model, camera and picked selection are untouched.
• CodeMirror colors are CSS variables (one theme serves both); the loading veil's canvas reads its inks from the page.

Checked (ego-browser)

• Light and dark screenshots of the same state: model with a picked face and the selection cell, module view with controls, log dock with an error, loading veil, workspace tab. Dark is unchanged.
• Toggle cycle and persistence across a reload; system follows the emulated OS theme live; switching keeps the camera and the selection overlay.
• The light viewport background matches the page (254 vs 255 after tone mapping).
• npm run build (tsc + vite) and pytest test/test_editor.py pass.

Notes

• Not exercised: a real OS theme change on a physical browser (headless emulation only fires the change event after a frame is rendered).
• The LazyLoader ... 'cryptography' package is not installed warning still shows in the first cell's output; unrelated to this change.