fix(web): port #11594 to V2 — the chat width setting widens V2's chat lane

Main's chat width setting sets --chat-max-width, which main's timeline and
composer read. V2 sizes the same surfaces (chat-content-lane, the
composer, and ChatCanvas's width probe that drives the preview layout)
with --chat-content-max-width, so after the merge the setting was saved
but changed nothing. Wide and Full now override V2's token; Comfortable
keeps V2's default (46rem, close to main's 48rem).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: juliusmarminge

apps/web/src/index.css

@@ -2019,17 +2019,14 @@ code {
   --diff-deletion-foreground: light-dark(var(--color-red-600), var(--color-red-400));
 }
 
-/* Chat timeline and composer column width, set from the Chat width setting. */
-:root {
-  --chat-max-width: 48rem;
-}
-
+/* Chat width setting. Comfortable keeps the default --chat-content-max-width,
+   which sizes the timeline lane, the composer, and ChatCanvas's width probe. */
 :root[data-chat-width="wide"] {
-  --chat-max-width: 72rem;
+  --chat-content-max-width: 72rem;
 }
 
 :root[data-chat-width="full"] {
-  --chat-max-width: 100%;
+  --chat-content-max-width: 100%;
 }
 
 :root[data-diff-color-scheme="blue-orange"] {