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/setup.go

@@ -18,11 +18,7 @@ func (h *Handlers) HandleSetupGET() http.HandlerFunc {
 	return func(writer http.ResponseWriter, request *http.Request) {
 		data := h.addGlobals(map[string]any{}, request)
 
-		err := tmpl.ExecuteTemplate(writer, "setup.html", data)
-		if err != nil {
-			h.log.Error("template execution failed", "error", err)
-			http.Error(writer, "Internal Server Error", http.StatusInternalServerError)
-		}
+		h.renderTemplate(writer, tmpl, "setup.html", data)
 	}
 }
 
@@ -62,7 +58,7 @@ func (h *Handlers) renderSetupError(
 		"Username": username,
 		"Error":    errorMsg,
 	}, request)
-	_ = tmpl.ExecuteTemplate(writer, "setup.html", data)
+	h.renderTemplate(writer, tmpl, "setup.html", data)
 }
 
 // HandleSetupPOST handles the setup form submission.