End-to-end: harness Playwright suite green (14 pass, 1 skip)

After the React-codegen rework, ran the full e2e harness against the
docker-stack backend. Surfaced and fixed real friction:

mizan-base/src/index.ts (kernel):
- MizanError now parses both error envelopes — the FastAPI shape
  ({"error": {"code", "message", "details"}}) and the Django shape
  ({"error": true, "code", "message", "details"}). Exposes .code and
  .details on the thrown error so consumer code can branch on them.
  This was needed for the harness's `instanceof MizanError && error.code
  === 'NOT_FOUND'` pattern to work; the previous MizanError only carried
  status + raw body, leaving callers to parse the body themselves.

examples/django-react-site/Dockerfile.test:
- Backend image now copies and installs cores/mizan-python before
  installing mizan-django (which imports from mizan_core after the
  Layer 1 extraction).

harness/src/fixtures.tsx:
- useRun helper updated for the new mutation-hook shape: pulls
  { mutate } off the hook result instead of treating the hook return
  as a callable. Same for ValidationError fixture.

mizan.spec.ts:
- DjangoError → MizanError (kernel error class is backend-agnostic).
- Form tests removed (forms codegen deferred per Blazr scope).
- Channel test marked test.skip (channels deferred per Blazr scope).

.gitignore: ignore Playwright test-results/.

Final verification across all surfaces:
- mizan-core unit:       15/15
- mizan-django unit:     348 pass, 21 skip
- mizan-fastapi unit:    11/11
- mizan-ts edge-compat:  34/34 (cross-language HMAC pin)
- harness e2e (Playwright): 14/15 (1 skip = channels deferred)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontends/mizan-base/src/index.ts

@@ -9,8 +9,24 @@
 // === Error ===
 
 export class MizanError extends Error {
+    public code: string
+    public details?: unknown
+
     constructor(public status: number, public body: string) {
         super(`Mizan call failed (${status})`)
+        // Two envelope shapes are tolerated:
+        //   FastAPI:  {"error": {"code", "message", "details"}}
+        //   Django:   {"error": true, "code", "message", "details"}
+        try {
+            const parsed = JSON.parse(body)
+            const err = parsed?.error
+            const source = typeof err === 'object' && err !== null ? err : parsed
+            this.code = source?.code ?? `HTTP_${status}`
+            this.details = source?.details
+            if (source?.message) this.message = source.message
+        } catch {
+            this.code = `HTTP_${status}`
+        }
     }
 }