chore(frontend): controlled dependency migration after failed grouped bumps

[blueberrycongee]

Background

Two grouped Dependabot PRs are currently failing CI and are not safe to merge directly:

• build(deps): bump the npm-prod group with 24 updates #113 (npm-prod group)
• build(deps-dev): bump the npm-dev group with 12 updates #114 (npm-dev group)

Current hard failures:

1. tailwindcss v4 PostCSS plugin split (@tailwindcss/postcss required).
2. Type incompatibility around editor RefObject typing in src/editor/Editor.tsx.

Why this issue exists

Grouped dependency bumps hide breaking changes and produce low-signal CI noise. For this repo, frontend upgrades need controlled, staged migration.

Scope

• Fix editor ref typing to satisfy strict TypeScript checks.
• Migrate Tailwind/PostCSS pipeline to v4-compatible setup.
• Split upgrades into reviewable PRs (tooling, UI runtime, test/build chain).
• Ensure CI green (frontend, npm-audit, rust checks).

Delivery plan

1. Land Tailwind/PostCSS migration in a focused PR.
2. Land editor typing fix and related TS config adjustments in a focused PR.
3. Re-open dependency upgrades in smaller batches only after baseline is green.

Definition of done

• Main branch CI remains green after each step.
• No grouped Dependabot PR with mixed breaking changes.
• Frontend build and lint pass locally and in CI.