fix(app): keep contribution editor toolbar visible below navbar

[madsykle]

Summary

The MDXEditor toolbar on the content step of the contribution flow had position: sticky; top: 1rem; z-index: 50, same z-index as the sticky navbar. So on scroll it ended up hovering inside the navbar's stacking context instead of sitting below it. Fixed by pinning it to top: 65px, which is the offset every other sidebar already uses for "below navbar," and dropping its z-index under the navbar's. Fullscreen mode still uses top: 1rem since the navbar isn't shown there.

Also fixed the background. hsl(var(--card) / 0.75) doesn't work because --card is already a hex value, not raw HSL channels, so that CSS was invalid and got dropped silently, leaving the toolbar transparent. Switched it to var(--card) directly.

Type of change

• Bug fix
• Feature
• Refactor (no behavior change)
• Documentation
• Build / tooling / CI
• Other: _________

Verification

• pnpm -r typecheck passes
• pnpm -r build passes
• Manually verified in a browser (for UI / behavior changes)
• Followed the app layout conventions
• No new dependencies, or new dependencies are justified and AGPL-compatible
• Change does not violate any non-negotiable principle
• Documentation only, no changes to the codebase, so no tests required

Closes #286