fix: move dark editor background rule outside @layer base for correct cascade priority

[j4rviscmd]

Summary

Fix dark mode editor background not matching the app's dark theme.

Problem

The .dark .bn-editor CSS rule was declared inside Tailwind's @layer base, which has lower specificity than BlockNote's default styles defined outside any @layer. As a result, BlockNote's --bn-colors-editor-background (#1f1f1f) took precedence over the app's --background token in dark mode, causing the editor background to appear lighter than the surrounding UI.

Changes

• Moved .dark .bn-editor { background-color: var(--background); } from inside @layer base to outside it, so the rule wins the CSS cascade over BlockNote's defaults
• Added a detailed comment explaining the cascade behavior and why the rule must remain outside @layer base

Testing

• Verified dark mode editor background now matches the app's dark theme (oklch(0.145 0 0))
• Verified light mode is unaffected
• npm run build passes
• cargo build passes