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)
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