web-ui: single-header multiview panes (#513)

Multiview panes rendered two stacked headers: the pane chrome plus the hosted chat's own session top bar. Panes now show one header — the pane tab carries a scope/title breadcrumb, the inner chat suppresses its own top bar when hosted in a pane, and the per-context tools (crons, files, apps, skills, memory, keychain) plus the focus-over-grid action move behind a  overflow menu next to the pane's + button, leaving  and full-screen as the only inline chrome.

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