Fix: Include themeVariables in preview cache key

[taylortom]

Fixes #204

Fix

• Add a varsHash component to the preview prebuilt cache key — a deterministic 12-char hash over themeVariables and customStyle — so two courses sharing the same (pluginHash, theme, menu) but with different presets or custom LESS no longer collide on the cache.
• Hash the raw mongo course.data before preBuildHook/applySchemaDefaults mutate it, so the same course consistently produces the same hash across runs. This also matches the eager prebuild's "no customisation" view, so default-themed real courses correctly share its cache entry.
• Extend BuildCache.{getPath,has,populate,restore} to take and append the varsHash to the cache directory name.
• Update prebuildCache to populate with the hash of empty inputs (DEFAULT_VARS_HASH).

Testing

• node --experimental-test-module-mocks --test 'tests/**/*.spec.js' passes (28 tests, including new utils-computeVarsHash.spec.js and extended BuildCache.spec.js).
• npx standard is clean.
• Manual: preview course A with default preset, then preview course B (same plugins/theme/menu) with a changed preset colour — course B's CSS now reflects its own preset, not course A's.
• Manual: re-preview course A — still served from the original (default-vars) cache entry, no rebuild.

[github-actions]

🎉 This PR is included in version 3.1.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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