feat: add diagnostic web app shell

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

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+# Phase 7 Context: Web App Shell and Diagnostic Start
+
+**Status:** Ready for execution
+**Started:** 2026-04-26
+
+## Goal
+
+Serve the first browser UI from the Go backend and let a job seeker start
+diagnostic practice without API tooling.
+
+## Requirements
+
+- WEB-01: User can open a web app served by the Go service.
+- WEB-02: User can create a diagnostic interview session from the web app.
+- WEB-03: User can answer a diagnostic question and see rubric feedback.
+
+## UX Direction
+
+Visual thesis: quiet interview coaching workspace, dense but readable, with one
+clear green accent for action and readiness.
+
+Content plan:
+
+- left rail: learner setup
+- main workspace: active diagnostic questions and answer input
+- right context: grading feedback and evidence
+
+Interaction thesis:
+
+- loading states on API actions
+- inline error region
+- selected question state and answer result refresh
+
+## Out of Scope
+
+- Full progress view.
+- Material/ontology workspace.
+- Authentication.