feat(chat): center chat reading column and full-height context sidebar

Replaces the full-variant flex layout with a single CSS grid owning the
messages column, input bar, and context sidebar. Messages and input bar
share a centered ~820px reading track (--chat-reading-width token) so
the mic button can't get pushed off-screen on wide monitors, and the
context sidebar spans grid-row 1/-1 so it runs the full height from the
chat header down to the bottom edge instead of stopping above the input
bar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/assets/theme.css

@@ -54,6 +54,8 @@
   --page-max-width: 1200px;
   --page-padding-x: 1rem;
   --sidebar-width: 260px;
+  --chat-reading-width: min(820px, 100%);
+  --chat-context-sidebar-width: 220px;
   --color-accent-warm: #b8860b;
   --color-accent-warm-light: #d4a017;
   --color-primary-solid: #7c3aed;