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.

client/src/components/lanes/__tests__/LaneChangesPanel.test.tsx

@@ -1308,6 +1308,27 @@ describe("a merge in progress (issue #45)", () => {
     );
   });
 
+  it("opens a conflicted file in the conflict view, not the diff", async () => {
+    vi.mocked(api.lanes.git).mockResolvedValue(merging(1));
+    vi.mocked(api.lanes.changes).mockResolvedValue(conflicted);
+    vi.mocked(api.lanes.gitFile).mockResolvedValue({
+      available: true,
+      path: "a.txt",
+      rev: "work",
+      lines: ["<<<<<<< HEAD", "mine", "=======", "theirs", ">>>>>>> feat"],
+      truncated: false,
+      binary: false,
+      sha: "s1",
+    } as never);
+    const user = userEvent.setup();
+    render(<LaneChangesPanel laneId={3} />);
+    await user.click(
+      within(await screen.findByTestId("lane-changes-conflicts")).getByText("a.txt")
+    );
+    expect(await screen.findByTestId("lane-conflict-view")).toHaveTextContent("Theirs (feat)");
+    expect(api.lanes.gitFile).toHaveBeenCalledWith(3, "a.txt", "work");
+  });
+
   it("completes the merge once nothing is conflicted, and aborts only on a second click", async () => {
     vi.mocked(api.lanes.git).mockResolvedValue(merging(0));
     const user = userEvent.setup();