Auto-select branch containing active snapshot on reload

When activeBranchSelections is empty (after reload or tab switch),
selectBranchContaining is called with the effective active ID so the
highlighted branch matches the active snapshot ring.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/snapshot_manager.js

@@ -3675,6 +3675,10 @@ function buildTimeline() {
         let tree = null;
         if (branchingEnabled) {
             tree = buildSnapshotTree(allRecords);
+            // Auto-select branch containing the active snapshot on first render
+            if (activeBranchSelections.size === 0 && effectiveActiveId && tree.byId.has(effectiveActiveId)) {
+                selectBranchContaining(effectiveActiveId, tree);
+            }
         }
 
         // ── Expanded mode: one row per branch ──