Move spring-editor to monorepo with aligned versions

[Copilot]

This PR moves the spring-editor Vite application from the standalone remotion-dev/spring-editor repository into the main Remotion monorepo as @remotion/spring-editor, aligning all dependency versions with the packages/convert package.

Key Changes

Package Structure

• Created new packages/spring-editor package with proper monorepo structure
• Updated package name from spring-editor to @remotion/spring-editor
• Added package to workspace and turbo build system

Version Alignments

• Vite: 6.3.5 → 5.4.19 (aligned with convert package)
• React: 18.2.0 → 19.0.0 (aligned with convert package)
• ESLint: 8.52.0 → 9.19.0 (aligned with convert package)
• TailwindCSS: 3.3.3 → 4.1.10 (aligned with convert package)
• All @remotion/* packages now use workspace:* references

Configuration Updates

• Updated ESLint config to use new v9 flat config format (eslint.config.mjs)
• Aligned PostCSS config with monorepo standards
• Updated Vite config to follow monorepo conventions
• Added turbo.json configuration for proper build caching

Code Quality

• Fixed ESLint errors to match monorepo standards
• Applied Prettier formatting to all source files
• Only 2 TODO comment warnings remain (non-breaking)

Verification

The package now:

• ✅ Builds successfully with bunx turbo make --filter="@remotion/spring-editor"
• ✅ Integrates with the monorepo's turbo build system and caching
• ✅ Passes ESLint with monorepo standards
• ✅ Uses aligned dependency versions across the stack

The spring editor functionality remains unchanged - this is purely a migration to consolidate the codebase and standardize tooling.

Fixes #5429.

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Name	Status	Preview	Comments	Updated (UTC)
bugs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 19, 2025 9:53am
remotion	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 19, 2025 9:53am