feat(hub): legal information link at the foot of the sidebar

A small link to the hub's legal pages, pinned to the bottom of the
sidebar once signed in. It opens in a new tab: the desktop application
refuses to navigate away from its interface and hands a new window to
the system browser, and in a browser it keeps the session on screen. The
address comes from hubBase(), so it is the legal pages of the hub in use.

The sidebar now sticks under the navigation bar at the window's height;
otherwise, on a long file list, the link would sit at the bottom of the
page. The music bar, pinned to the bottom of the window as well,
publishes its height through useStickyBand as --music-bar-h and the
sidebar stops above it. On a phone the slide-out panel does the same.

test_sidebar_legal_measured.py measures the real stylesheet: the link at
the bottom of the window on a 3000px page, and above the music bar, at
phone and desktop widths. Checked signed in on a local hub in Chrome and
Firefox 155.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D9MCBBWSm9GhBESmqzJxNy

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