feat(merge): additive conflict accept — stack ours/theirs in click order

Replace the single-side replace (and the short-lived accept-both button) with
additive accepts: each result conflict region starts EMPTY (thin marker bar), and
the gutter controls append a side in click order — > adds ours, < adds theirs
(first pick on top, next below), x clears. Controls stay visible after the first
pick so both sides can be stacked; empty/unresolved regions render a marker so they
stay visible. en/de keys updated; Ui 128 + Localization 16 green.

src/ClaudeDo.Localization/locales/en.json

@@ -406,9 +406,9 @@
     "binaryHint": "Binary files can't be merged here — abort and resolve them in your editor:",
     "prevConflict": "Previous conflict (Shift+F8)",
     "nextConflict": "Next conflict (F8)",
-    "acceptOurs": "Accept ours into result",
-    "acceptTheirs": "Accept theirs into result",
-    "acceptBoth": "Accept both (ours, then theirs)",
+    "acceptOurs": "Add ours (stacks in click order)",
+    "acceptTheirs": "Add theirs (stacks in click order)",
+    "clearConflict": "Clear this conflict (start over)",
     "continue": "Resolve & continue",
     "abort": "Abort merge"
   },