ui : fix models page navigation and Discover gating
Record the models page's way back after every navigation, so Back,
a typed URL or a reload no longer return to a stale chat. Escape
closes the detail pane, then leaves the page. Gate every Discover
entry point on one shared check, recheck the API key on /models,
skip the duplicate router fetch on a direct load, and share the
route load between pages.
Assisted by: Claude (Opus 5.5 Medium)
