Add certificate issuance, CSR signing, and cert listing to web UI

- Add SignCSR RPC to v2 CA proto and regenerate; implement handleSignCSR
  in CA engine and caServer gRPC layer; add SignCSR client method and
  POST /pki/sign-csr web route with result display in pki.html
- Fix issuer detail cert listing: template was using map-style index on
  CertSummary structs; switch to struct field access and populate
  IssuedBy/IssuedAt fields from proto response
- Add certificate detail view (cert_detail.html) with GET /cert/{serial}
  and GET /cert/{serial}/download routes
- Update Makefile proto target to generate both v1 and v2 protos

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

web/templates/cert_detail.html

@@ -0,0 +1,36 @@
+{{define "title"}} - Certificate: {{.Cert.Serial}}{{end}}
+{{define "content"}}
+<div class="page-header">
+    <h2>Certificate: {{.Cert.CommonName}}</h2>
+    <div class="page-meta">
+        <a href="/pki/issuer/{{.Cert.Issuer}}">&larr; Issuer: {{.Cert.Issuer}}</a>
+        &ensp;&middot;&ensp;
+        <a href="/pki/cert/{{.Cert.Serial}}/download">Download Certificate (PEM)</a>
+    </div>
+</div>
+
+<div class="card">
+    <div class="card-title">Certificate Details</div>
+    <table class="kv-table">
+        <tbody>
+            <tr><th>Serial</th><td><code>{{.Cert.Serial}}</code></td></tr>
+            <tr><th>Common Name</th><td>{{.Cert.CommonName}}</td></tr>
+            <tr><th>Issuer</th><td>{{.Cert.Issuer}}</td></tr>
+            <tr><th>Profile</th><td>{{.Cert.Profile}}</td></tr>
+            {{if .Cert.SANs}}
+            <tr><th>SANs</th><td>{{range $i, $san := .Cert.SANs}}{{if $i}}, {{end}}{{$san}}{{end}}</td></tr>
+            {{end}}
+            <tr><th>Issued By</th><td>{{.Cert.IssuedBy}}</td></tr>
+            <tr><th>Issued At</th><td>{{.Cert.IssuedAt}}</td></tr>
+            <tr><th>Expires At</th><td>{{.Cert.ExpiresAt}}</td></tr>
+        </tbody>
+    </table>
+</div>
+
+<div class="card">
+    <div class="card-title">Certificate PEM</div>
+    <div class="form-group">
+        <textarea rows="12" class="pem-input" readonly>{{.Cert.CertPEM}}</textarea>
+    </div>
+</div>
+{{end}}