fix: buffer template execution to prevent corrupt HTML responses (closes #42)

Add renderTemplate helper method on Handlers that renders templates to a
bytes.Buffer first, then writes to the ResponseWriter only on success.
This prevents partial/corrupt HTML when template execution fails partway
through.

Applied to all template rendering call sites in:
- setup.go (HandleSetupGET, renderSetupError)
- auth.go (HandleLoginGET, HandleLoginPOST error paths)
- dashboard.go (HandleDashboard)
- app.go (HandleAppNew, HandleAppCreate, HandleAppDetail, HandleAppEdit,
  HandleAppUpdate, HandleAppDeployments)

internal/handlers/handlers.go

@@ -2,6 +2,7 @@
 package handlers
 
 import (
+	"bytes"
 	"encoding/json"
 	"log/slog"
 	"net/http"
@@ -18,6 +19,7 @@ import (
 	"git.eeqj.de/sneak/upaas/internal/service/auth"
 	"git.eeqj.de/sneak/upaas/internal/service/deploy"
 	"git.eeqj.de/sneak/upaas/internal/service/webhook"
+	"git.eeqj.de/sneak/upaas/templates"
 )
 
 // Params contains dependencies for Handlers.
@@ -80,6 +82,28 @@ func (h *Handlers) addGlobals(
 	return data
 }
 
+// renderTemplate executes the named template into a buffer first, then writes
+// to the ResponseWriter only on success. This prevents partial/corrupt HTML
+// responses when template execution fails partway through.
+func (h *Handlers) renderTemplate(
+	writer http.ResponseWriter,
+	tmpl *templates.TemplateExecutor,
+	name string,
+	data any,
+) {
+	var buf bytes.Buffer
+
+	err := tmpl.ExecuteTemplate(&buf, name, data)
+	if err != nil {
+		h.log.Error("template execution failed", "error", err)
+		http.Error(writer, "Internal Server Error", http.StatusInternalServerError)
+
+		return
+	}
+
+	_, _ = buf.WriteTo(writer)
+}
+
 func (h *Handlers) respondJSON(
 	writer http.ResponseWriter,
 	_ *http.Request,