docs: start frontend mvp milestone

openspec/changes/frontend-mvp/design.md

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+# Design: Frontend MVP
+
+## Approach
+
+Use a dependency-light web app served by the Go backend. The first UI can be
+plain HTML/CSS/JavaScript so the repo avoids frontend build tooling until the
+interaction model is proven.
+
+## UX Principles
+
+- First screen is the working tutor experience, not a marketing landing page.
+- Keep the UI quiet, operational, and focused on repeated practice.
+- Show evidence-backed feedback and next actions after each loop.
+- Use clear loading, empty, and error states.
+
+## Boundaries
+
+In scope:
+
+- Diagnostic practice.
+- Learner memory and readiness display.
+- Material ingestion and ontology inspection.
+- Teaching asset prompt candidates.
+
+Out of scope:
+
+- Authentication.
+- Multi-user persistence.
+- Payment/subscription.
+- Real provider image generation.