One merge window, not fifteen

show_conflict_summary() built a fresh ConflictSummaryDialog every time and
only reassigned self._conflict_dialog — the old dialog was still a child of
the window, so it stayed on screen. check_for_external_changes() runs
resolve_conflicts() on every Syncthing watch tick, so that was one window per
merge, forever. Fifteen had piled up on trav's desktop.

The dialog is now a session-long log: add_event() folds each merge in as its
own timestamped entry, newest first, and the restore button names the merge
it would undo. MainWindow reuses the open dialog and clears its reference on
finished, so closing it lets the next merge open a fresh one instead of
touching a deleted C++ object.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M8Mzze7shr5pZoEgKQU5NW

TASKS.md

@@ -38,9 +38,11 @@ performance half. This is the correctness half.
       never conflict, effective count = base + sum of journals, and `library.json`
       stops being rewritten every 3 s during playback — which is what generates
       the conflicts in the first place.
-- [ ] **Quiet the merge dialog.** Lossless merges (counts and dates only) should
-      be a status-bar line, not a window; keep the dialog for lossy cases, and
-      reuse one instance so six can never stack up again.
+- [x] **Quiet the merge dialog.** One dialog per session: each merge is folded
+      in as a timestamped entry (newest first) instead of opening another
+      window. Fifteen had stacked up. Kept the window for every merge rather
+      than demoting lossless ones to the status bar — with journals landing,
+      a merge stops being routine and is worth seeing.
 - [ ] Consider a `.stignore` for `.resolved` so merge backups stop syncing
       (Round 35 bounded the folder to 10 snapshots, but it still replicates).