Add a three-way merge view for conflicted files

[geevensingh]

Problem

The file list already surfaces a "Conflicted" section, but conflicted files
render in the regular two-pane diff view, which doesn't show all three
sides (ours / theirs / base) that conflict resolution needs. Users have to
drop to another tool to actually resolve.

Proposal

For files in conflicted state, switch the diff pane to a three-pane (or
three-track) layout:

• Base — the merge base blob
• Ours — current branch's version (HEAD)
• Theirs — the incoming branch's version (MERGE_HEAD)
• Result — the working-tree version with conflict markers, editable

Provide per-hunk accept ours / accept theirs / accept both
buttons that write back to the working tree (the same write surface as the
existing stage / unstage / revert actions).

Open questions

• Whether to surface all three source blobs simultaneously or stack them as
  two diffs (base→ours and base→theirs). Stacked is less screen real
  estate but loses the at-a-glance "where do ours and theirs disagree"
  signal.
• How to handle conflicts in non-text files (binary, image,
  deleted-on-one-side, both-added with different content).
• Whether the "Result" pane is editable in DiffViewer or whether the user
  edits in their own editor and DiffViewer just shows the current state.
  Editable is much more ambitious; read-only-with-accept-buttons is the
  smaller MVP.

Acceptance

• Conflicted files open in the three-way view (whatever layout is chosen).
• "Accept ours / theirs / both" produce the correct working-tree content
  and clear the conflict markers from the result.
• (If editable) saving from the result pane writes back to disk.
• Tests cover the merge-resolution path in GitWriteService (or a new
  sibling service).

Notes

Tier 2 from the feature brainstorm. Scope: large — the most ambitious item
in this batch. Strongly recommend splitting into sub-issues: view-only
three-way first, accept-buttons second, editable result third.