feat: Streamline VST3 companion app installation and plugin scanning UX

[ChuxiJ]

Context

From DAW tech stack research: We have a production-level VST3 host via Rust companion app — a major differentiator vs other web DAWs (Soundtrap, BandLab have zero plugin support). However, the installation and plugin discovery UX needs to match the quality of native DAWs like Bitwig or Ableton where plugins "just work."

Goal

Make the VST3 companion app setup as frictionless as possible — from download to first plugin load.

Acceptance Criteria

• One-click companion app download/install flow from within the DAW UI
• Auto-detect companion app status (running/not running/outdated)
• Plugin scan progress UI with categorization (instruments, effects, by vendor)
• Clear error states: companion not found, plugin failed to load, version mismatch
• Plugin browser with search, filter by type/vendor, favorites
• First-time setup wizard/guide for new users
• Document supported platforms (Windows, macOS, Linux)

Strategic Context

Industry benchmark: Bitwig/Ableton auto-scan on startup, categorized plugin browser
Our advantage: Only web DAW with native VST3 support
Risk: UX friction could negate the technical advantage

Part of: DAW Best Practices Research Series

[ChuxiJ]

Priority Update: P0 → P2

VST3 companion UX only affects users who need VST3 plugins — a subset of the user base. Basic DAW interactions (copy/paste, undo, error handling) must work perfectly first.

Re-prioritized as part of Experience Polish Series.

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