feat: add diagnostic web app shell

.planning/phases/007-web-app-shell-diagnostic-start/007-SUMMARY.md

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+# Phase 7 Summary
+
+**Status:** Complete
+**Completed:** 2026-04-26
+
+## Delivered
+
+- Added embedded web app serving from the Go backend.
+- Added `GET /` app shell and `/assets/*` static asset handling.
+- Built dependency-light HTML/CSS/JavaScript UI for diagnostic practice.
+- Added setup form for user id, target role, stack, and timeline.
+- Added real API-backed diagnostic session creation.
+- Added question selection, answer submission, and rubric feedback rendering.
+- Added loading, error, empty, and selected-question states.
+- Added web app route and asset tests.
+
+## Verification
+
+```powershell
+gofmt -w cmd internal
+go test ./...
+openspec validate frontend-mvp --strict
+openspec validate bootstrap-job-tutor-platform --strict
+```
+
+Additional smoke check:
+
+- `GET /` returned the app shell.
+- `GET /assets/app.js` returned the browser script.
+- Diagnostic session creation and answer grading succeeded through the same
+  server used by the app.
+
+## Deferred
+
+- Progress panels for memory/readiness/next challenge.
+- Material and asset workspace.
+- Browser screenshot audit.