Add a drag handle on the left edge of the docked Ask Fabro panel. The
user can widen the panel up to 2x its default width and no narrower than
the default. The chosen width persists across open/close.
An `isResizing` flag on the layout context lets `<main>` and the steer
bar drop their width transitions during a drag so the layout tracks the
cursor instead of trailing it.
Also bundles in-progress sidebar wiring: a collapsed tool-call summary
renderer and the remark-gfm dependency for GitHub-flavored Markdown.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
