fix(layout): bound the grid tracks so wide content cannot widen the shell

[github-actions]

The chat overflow persisted after the previous attempt because I fixed the
wrong layer. min-width:0 on .chat-messages / .chat-msg was correct in principle
but irrelevant: the overflow originates ABOVE them.

A 1fr grid track has an automatic minimum of min-content, so a track cannot
shrink below its widest descendant. One wide markdown table therefore forced
the content column past the viewport, and every descendant inherited the
over-wide box. .content (overflow:hidden, min-width:0) and .ws-panel
(min-width:0) were already correct — the constraint simply never reached them.

Bounded with minmax(0, ...):
.view-shell sidebar + content column
.ws-grid-single columns and rows
.ws-grid-split-2x1 columns, rows
.ws-grid-split-1x2 columns, rows
.ws-grid-t-top columns, rows

This is also why single-pane looked WORSE than split: the content column is the
full window there, so the overrun ran off screen entirely instead of being
bounded by the neighbouring pane.

Also removes the overflow-x:hidden I added to .chat-messages last time. It hid
the symptom and made it worse — the text was clipped with no way to scroll to
what was cut off. With the tracks bounded, a wide table scrolls inside its own
box (.chat-msg-body table) as originally intended.

The visual.spec.ts pixel baselines cover the app shell, so a regression here is
catchable in CI in a way the sidebar flash never was.

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Opened automatically after the tiers check passed on 5a27c435.

The check is attached to this branch head, so this is mergeable now.