focus-trap: focus the trapped container synchronously on mount instead of
only after a deferred rAF. The keydown listener is node-scoped, so an Escape
pressed before the rAF moved focus into the sheet landed on an element outside
the node and was silently dropped — a real keyboard-a11y gap (open a sheet,
immediately press Escape → nothing happened). The rAF still refines focus to
the first control once laid out.
recover: replace the `window.history.length > 1` back-chevron heuristic with
SvelteKit's afterNavigate `from` signal. history.length is 2 on a fresh-tab
deep link (about:blank + page), so history.back() landed on the blank entry.
`from` is null only on a full-page load, so deep-linked users now correctly
fall back to /join (or /feed when authed).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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