feat: add React diagnostics tooling

[tanvesh01]

Summary

• add React Scan dev instrumentation before the React root mounts
• add dev scripts for React Scan and React Doctor checks
• add tests for diagnostics mode/env flag behavior

Verification

• bun test src/lib/react-diagnostics.test.ts
• bun test
• bun x tsc --noEmit
• bun run doctor:changed --json --json-compact
• bun x react-doctor --staged --scope files --no-score --json --json-compact
• bun run dev:scan -- --host 127.0.0.1, then curl http://127.0.0.1:1420/ and transformed /src/lib/react-diagnostics.ts
• git diff --cached --check
• bun install --frozen-lockfile

Note: did not run forbidden build commands (bun run build, cargo build, tauri build).

[tanvesh01]

Frontend build verified after request:

• bun run build ✅
• Output: tsc && vite build completed successfully (✓ built in 5.86s).
• Vite emitted the existing large chunk warning for chunks >500 kB, but the build exited 0.

[tanvesh01]

This was generated by AI during repo maintenance.

Close candidate: stale merge conflict (P3)

This PR matches the conservative P3 stale-conflict signal:

• mergeStateStatus is DIRTY, so it cannot merge cleanly into main as-is.
• The only commit is 498c798 (feat: add React diagnostics tooling) from 2026-06-30 12:14 UTC.
• The last PR activity/comment was 2026-06-30 12:17 UTC, more than 14 days ago.
• There are no assigned reviewers, no review requests, and no submitted reviews.

Labeling close-candidate for maintainer review. I am not closing the PR.