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>

Makefile

@@ -8,6 +8,9 @@ proto:
 	protoc --go_out=. --go_opt=module=git.wntrmute.dev/kyle/metacrypt \
 		--go-grpc_out=. --go-grpc_opt=module=git.wntrmute.dev/kyle/metacrypt \
 		proto/metacrypt/v1/*.proto
+	protoc --go_out=. --go_opt=module=git.wntrmute.dev/kyle/metacrypt \
+		--go-grpc_out=. --go-grpc_opt=module=git.wntrmute.dev/kyle/metacrypt \
+		proto/metacrypt/v2/*.proto
 
 metacrypt:
 	go build $(LDFLAGS) -o metacrypt ./cmd/metacrypt