feat(canvas): independent background selection + custom color

[dani-polani]

What

Decouple the canvas from the visual style so any style can render on any background. Answers the user request to use a style's word treatment (e.g. Bauhaus blocks) on a different canvas.

• The Style panel's Canvas section is now a background grid available for every style (light, dark + the 12 style canvases), not only Classic.
• New Custom tile paints the diagram on a solid user-chosen color via the native color picker; text color and light/dark chrome are derived from the color's luminance (no separate text control needed).
• The main Style picker still selects the whole style; picking a style resets the background override to that style's default — the same pattern palette already uses.

How

• styles.ts: BackgroundId axis + BACKGROUNDS catalog derived from the style canvases; resolveCanvas() (shared by all render/export sites), computeSolidCanvas() for custom, backgroundExport(id,…) (style wrapper kept for callers/tests).
• schema.ts + compact-v4.ts: additive backgroundId / backgroundCustomColor, carried in the share link (bi, bc) only when set/active.
• AlignmentPreview / TokenView / ExportMenu / svg.ts: render + export the resolved canvas; plain light/dark keep the preview-frame CSS path, custom is a caller-filled solid.
• StylePanel.svelte: background grid + custom color-input tile (native swatch styled to match the sibling tiles).

Compatibility

Additive fields only — an old share link with no backgroundId renders exactly as before. Note: Classic dark chosen via the grid is now an override, so it resets on a style switch (consistent with the "don't pin tweaks" rule).

Verification

• 163 unit tests pass (13 added: catalog/resolver, export override incl. Bauhaus-on-Aurora and custom solid fill, serialization round-trips). Typecheck / lint / build clean.
• Driven live with Playwright: Bauhaus chips on the Aurora gradient; custom solid blue with auto-white text; a light-yellow custom flips auto-text to near-black; switching style resets the canvas.

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