docs: Publish web/desktop parity matrix and runtime degradation UI

[ChuxiJ]

Problem

#1529 remains unresolved. There is no published web/desktop feature parity matrix and runtime degradation UI is not guiding users when desktop-only capabilities are unavailable.

Root Cause

Implementation work focused on native capabilities first, while product-facing parity documentation and feature detection UX were deferred.

Solution

• Publish the web vs desktop feature matrix for audio engine, VST3/WAM, native file I/O, MIDI, packaging, and AI generation.
• Add runtime feature detection surfaces for unavailable desktop-only capabilities.
• Define degradation behavior and user-facing copy for browser mode.

Verification

• Review docs against current open/closed phase issues.
• Add tests for feature-detection helpers where applicable.
• Browser mode should explain unavailable native capabilities without broken controls.

Files to Touch

• docs/qa/story-matrix.md
• docs/*
• src/utils/tauri.ts
• src/components/*