feat(git): per-block conflict view with mine/theirs choices (#45)

Opening a conflicted file in the Changes panel now shows each conflict
block as its two sides, labelled from git's markers and tinted apart,
with Keep mine / Take theirs / Keep both per block (undoable, ^/v to
jump). Save and mark resolved writes the chosen text through the
whole-file editor's PUT /git/file (baseSha stale check) then runs
resolve done. The Merge conflicts group's buttons are text, and a
conflicted file is marked with a red ! in the Files tree.

Fix: ls-files --cached lists an unmerged path once per stage, so the
tree showed a conflicted file three times; lsFiles now dedupes.

server/lib/git-diff.js

@@ -760,7 +760,9 @@ async function lsFiles(dir, args) {
   const all = out.split("\0");
   // A complete listing ends in NUL (last token ""); a cut one ends mid-path.
   const cut = all.pop() !== "";
-  return { all: all.filter(Boolean), cut };
+  // Mid-merge, `ls-files --cached` prints a conflicted path once per stage
+  // (base, ours, theirs): one path is one file.
+  return { all: [...new Set(all.filter(Boolean))], cut };
 }
 
 /** What the explorer lists: tracked files plus untracked ones git does not