Hard-switch to the new React/FastAPI structure and remove legacy compatibility code

[quinCode]

Hard-switch to the new React/FastAPI structure and remove legacy compatibility code

Parent

#2

What to build

Complete the final switch to the new architecture. The React/Vite frontend and modular FastAPI backend should become the only supported UI source structure, and long-term legacy compatibility code should be removed.

This slice is not a feature migration by itself; it verifies the rewrite is complete and removes old source paths that would otherwise keep architecture drift alive.

Acceptance criteria

• Native static frontend source is no longer the primary source entry point.
• Long-term compatibility bridges, global shims, and old root-level scanner/viewer/runner source structure are removed or reduced to thin launch wrappers where explicitly required.
• FastAPI serves the new frontend build output.
• README, ADRs, and CONTEXT documentation describe the final source layout and integration path.
• Existing core workflows remain available in the new UI.
• All backend, frontend, CoreGateway, contract, boundary, deletion, and smoke tests pass.
• No React feature depends on raw core CLI output, YAML, progress event lines, or result file internals.

Blocked by

• Migrate catalog and schema-driven configuration as the first full feature module #7
• Migrate run creation and validation through CoreGateway #8
• Migrate task progress, events, and cancellation #9
• Migrate task history and config reuse #10
• Migrate result viewing and report viewing #11
• Add architecture enforcement tests and feature deletion tests #12