Migrate app to React Router 8 Framework Mode

[ceane]

Summary

Stacked on #59, this PR adds the first React Router 8 Framework Mode migration slice while preserving the existing SDR, Redux, WebSocket, Rust, WASM, 3D, React Flow, map, and legal-app boundaries.

What changed

• Upgraded to React Router 8.3.0 and React 19.2.7 with the Node >=22.22.0 baseline.
• Added Framework Mode config, generated route types, root document layout, public/authenticated route groups, route error boundaries, and compatibility redirects.
• Kept /get-started authenticated while giving it a lightweight route shell that does not initialize SpectrumProvider or the full application graph.
• Deferred the full app shell until authentication succeeds.
• Moved Demod/React Flow, 3D, map, and draw-signal providers behind route-scoped lazy boundaries.
• Preserved the legacy inner route tree as an incremental compatibility bridge.
• Added SSR-safe browser storage guards and Jest compatibility coverage for React Router 8's ESM package.
• Added npm run dev:router, npm run build:router, and npm run typegen.

Validation

• Framework production build: passed.
• Legacy frontend production build: passed.
• Typecheck: passed.
• Full Jest: 244 suites / 1,807 tests passed.
• Focused auth, onboarding, route, provider, and storage tests: passed.
• Rust and integration suites were run; existing backend/TX-monitor failures are documented in the local handoff.

Follow-up

React Router 8.3.0 rejects Lazy Route Discovery mode with ssr:false, so this SPA pass uses initial route discovery while retaining route-module code splitting. Enabling the lazy manifest requires the later SSR-capable deployment milestone. Browser-level Playwright lifecycle and deployment-version-skew coverage remain follow-up gates.

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