fix(chat): stop code blocks and tables forcing the message column wide

[github-actions]

Fourth attempt, and the first one based on measurement rather than reading CSS.
A Playwright probe on icecube rendering a deliberately wide message named the
culprit immediately:

PRE. scrollW=2864 boxW=2864
CODE.hljs scrollW=2834 boxW=2834
DIV.chat-msg-body boxW=2864
P. boxW=2864 <- prose inherited the width
TABLE. boxW=818 <- never the problem

The

 was 2864px and dragged .chat-msg-body and both 
 siblings with it,

which is why PROSE ran off the panel edge. The table was innocent throughout;

the three earlier fixes (flex min-width, grid minmax, table styling) all

targeted things that were not the cause, which is why none of them helped.
max-width:100% on the pre could never work: it resolves against

.chat-msg-body, which has no definite width of its own — it is a block in a

flex column being stretched BY this very child. Circular.
The fix breaks that cycle: width:0 keeps the element's intrinsic size out of

the parent's max-content calculation, min-width:100% expands it back to the

parent's resolved width, and overflow-x:auto scrolls the long line inside its

own box. Applied to both 
 and  — the table needed it too once

measured (bubble client=408 vs content=818). The inner  gets no width at

all; width:max-content there re-introduced exactly the 2834px the pre had just

given up.

Verified by re-running the probe after each change: .chat-msg-body and both 


went 2864 -> 818 -> 408, matching the bubble exactly, and only CODE still

"overflows" — inside its own scroll container, which is the design.
Ships with the probe as tests/e2e/flows/chat-overflow.spec.ts so this cannot

regress silently again. It renders a wide table, a 300-char unbroken URL and a

400-char code line, then asserts nothing in the panel has scrollWidth >

clientWidth, allowing TABLE/PRE/CODE to scroll within themselves. On failure it

prints every offender with its widths, so the next person gets the measurement

instead of a guess.

Opened automatically after the tiers check passed on 1a7b2d90.
The check is attached to this branch head, so this is mergeable now.