feat(merge): add accept-both control to the 3-pane conflict gutter

The between-pane gutter only offered single-side replace (accept ours / accept
theirs). Add an 'accept both' (⊕) control under the ours chevron that drops
ours-then-theirs into the result region, so a conflict can be combined in one
click instead of picking one side and hand-adding the other. en/de keys added.

src/ClaudeDo.Localization/locales/en.json

@@ -408,6 +408,7 @@
     "nextConflict": "Next conflict (F8)",
     "acceptOurs": "Accept ours into result",
     "acceptTheirs": "Accept theirs into result",
+    "acceptBoth": "Accept both (ours, then theirs)",
     "continue": "Resolve & continue",
     "abort": "Abort merge"
   },