Bolt-Era Main Container UI Analysis

Lifted from `augment-it-bolt/specs/MainContainerUI-Analysis.md`. The longest of the bolt-era analyses (~750 lines). Describes the app shell, authentication gate, routing, and layout structure of the bolt monolith. Most directly relevant to the rewrite's shell decision — does augment-it have one host shell (with mounted microfrontends), or is each app its own host?