review: address ds-review-bot findings on composer geometry

- Rewrite the 08-04 gutter-reservation note in place: the reservation is Chat's alone and the overlay branch points to the seat-width compensation note; the Testing section now describes the uncompensated control.
- Cross-reference the earlier rejection of the seat inset in the 08-12 note and record the engine-resolved-vs-fixed-8px drift as accepted residual cost instead of asserting Firefox resolves 8px.
- Pair --dsh-scrollbar-width with the mirrored ::-webkit-scrollbar rule and the compensation consumer in the scrollbar-styles gate; document the variable in the ui-theme README.
- Rename the e2e control to compareTabsWithoutCompensation and correct the vacuity-guard comment; rewrap the WorkspaceBrowser doc comment.

packages/client/ui-theme/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-theme/README.md
-README.md: df4d5e0370962bf6f2a8ac0a7b88d669225dc5c5
-README.zh.md: fb8e937979a5168803be78a49e1bc1e7eacd7a47
+README.md: c24f2ae5692fdd05134ce780d21569b87570d1c7
+README.zh.md: 518b8930027c88bd85139a8a7325cea1fd40b40c