fix: handle 204 No Content in fetchJSON, add FK-aware delete errors, v2 screenshots

Frontend: fetchJSON now returns empty object on 204 instead of failing
to parse empty body — fixes silent delete failures across all entities.
Added onError callbacks to owner/team delete mutations to surface errors.

Backend: owner and issuer delete handlers return 409 Conflict with
descriptive messages when FK constraints block deletion, instead of
generic 500.

Added 15 v2 dashboard screenshots, updated README screenshot section,
logo asset, page count references (18→full), and QA guide with FK
constraint test coverage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/features.md

@@ -818,7 +818,7 @@ All loops have configurable intervals via environment variables (`CERTCTL_SCHEDU
 
 ---
 
-## Web Dashboard (19 Pages)
+## Web Dashboard
 
 ### Overview
 The web dashboard is the primary operational interface for certctl. Built with **Vite + React 18 + TypeScript + TanStack Query v5 + Tailwind CSS 3 + Recharts**.
@@ -1168,7 +1168,7 @@ Each guide includes an evidence summary table mapping specific criteria to certc
 | Policies + violations | ✓ | ✓ | Shipped |
 | Profiles + crypto constraints | ✓ | ✓ | Shipped |
 | Revocation (RFC 5280, CRL, OCSP) | ✓ | ✓ | Shipped |
-| Dashboard + 19 pages | ✓ | ✓ | Shipped |
+| Full web dashboard | ✓ | ✓ | Shipped |
 | Observability (charts, metrics, stats) | ✓ | ✓ | Shipped |
 | REST API (91 endpoints) | ✓ | ✓ | Shipped |
 | MCP server (78 tools) | ✓ | ✓ | Shipped v2.1 |
@@ -1200,7 +1200,7 @@ Each guide includes an evidence summary table mapping specific criteria to certc
 | Category | Count |
 |----------|-------|
 | **API Endpoints** | 91 (under /api/v1/) |
-| **Dashboard Pages** | 19 |
+| **Dashboard** | Full web GUI |
 | **Issuer Connectors** | 4 (Local CA, ACME, step-ca, OpenSSL) |
 | **Target Connectors** | 5 (3 impl: NGINX, Apache, HAProxy; 2 stubs: F5, IIS) |
 | **Notifier Channels** | 6 (Email, Webhook, Slack, Teams, PagerDuty, OpsGenie) |