fix(web): stop the conversation column from scrolling sideways

The hero's backdrop ellipse is sized 1051/776 of the hero box so its blur
scales with the input card, which means it reaches past the column whenever
the column is narrower than the glow. `[data-conversation-scroll]` declared
only `overflow-y: auto`, and a box that scrolls in one axis computes the
other axis's initial `visible` to `auto` — so that bleed came back as a real
horizontal scrollbar, 24–95px of range across ordinary laptop widths.

Declare `overflow-x: hidden` on the column instead of leaving the second axis
to be derived. Clipping is unchanged (the box already clipped both axes); the
declaration withdraws only the bar and the user gesture.

tsconfig.host.json

@@ -29,6 +29,7 @@
     "apps/web/tests/hmr-live.e2e.ts",
     "apps/web/tests/seeded-history.e2e.ts",
     "apps/web/tests/sidebar-scrollbar.e2e.ts",
+    "apps/web/tests/conversation-column-overflow.e2e.ts",
     "apps/web/tests/code-mode-round.e2e.ts",
     "apps/web/tests/composer-draft-scroll.e2e.ts",
     "apps/web/tests/cordis-tool-round.e2e.ts",