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/dashboard.go

@@ -69,10 +69,6 @@ func (h *Handlers) HandleDashboard() http.HandlerFunc {
 			"AppStats": appStats,
 		}, request)
 
-		execErr := tmpl.ExecuteTemplate(writer, "dashboard.html", data)
-		if execErr != nil {
-			h.log.Error("template execution failed", "error", execErr)
-			http.Error(writer, "Internal Server Error", http.StatusInternalServerError)
-		}
+		h.renderTemplate(writer, tmpl, "dashboard.html", data)
 	}
 }