feat(history): highlight ahead/behind vs active upstream

Repo status updates for the management view are now refreshed in the
background after switching, without blocking the initial open flow.
The history graph and hover labels now distinguish local vs upstream
branches, visually marking commits that are ahead or behind.

- Add background fetch helper and shared status update logic
- Pass active upstream into HistoryPanel and render sync styling

src/app.css

@@ -2241,6 +2241,15 @@
   .graph-svg path.hidden-branch {
     opacity: 0.08;
   }
+  .graph-svg path.graph-segment-ahead {
+    stroke: #e0a040;
+    filter: drop-shadow(0 0 3px rgba(224,160,64,0.24));
+  }
+  .graph-svg path.graph-segment-behind {
+    stroke: #7aacff;
+    stroke-dasharray: 4 4;
+    filter: drop-shadow(0 0 3px rgba(122,172,255,0.24));
+  }
   .graph-dot {
     position: absolute;
     z-index: 2;
@@ -2258,6 +2267,14 @@
     opacity: 0.16;
     box-shadow: none;
   }
+  .graph-dot.ahead {
+    background: #e0a040;
+    box-shadow: 0 0 0 1px rgba(224,160,64,0.42), 0 0 10px rgba(224,160,64,0.16);
+  }
+  .graph-dot.behind {
+    background: #7aacff;
+    box-shadow: 0 0 0 1px rgba(122,172,255,0.46), 0 0 10px rgba(122,172,255,0.18);
+  }
   .graph-dot.tip { width: 14px; height: 14px; box-shadow: 0 0 0 1px var(--dot-color, #5a8cf8); }
   .graph-dot.merge {
     width: 15px;
@@ -2303,6 +2320,22 @@
     text-overflow: ellipsis;
     white-space: nowrap;
   }
+  .graph-hover-branches span.remote,
+  .branch-filter-option.remote {
+    border-color: rgba(122,172,255,0.32);
+    color: #bcd2ff;
+    background: rgba(31,43,72,0.88);
+  }
+  .graph-hover-branches span.ahead {
+    border-color: rgba(224,160,64,0.42);
+    color: #ffd99a;
+    background: rgba(58,42,20,0.94);
+  }
+  .graph-hover-branches span.behind {
+    border-color: rgba(122,172,255,0.46);
+    color: #c7dbff;
+    background: rgba(25,39,70,0.94);
+  }
   .graph-hover-branches svg {
     flex: 0 0 auto;
     color: #76d995;
@@ -2316,6 +2349,12 @@
     transition: background 120ms ease;
   }
   .graph-row + .graph-row .commit-body { border-top: 1px solid rgba(94,110,156,0.1); }
+  .graph-row.graph-ahead-row .commit-body {
+    box-shadow: inset 3px 0 0 rgba(224,160,64,0.72);
+  }
+  .graph-row.graph-behind-row .commit-body {
+    box-shadow: inset 3px 0 0 rgba(122,172,255,0.72);
+  }
   .graph-row:hover .commit-body { background: rgba(37,40,62,0.54); }
   .graph-row:hover .graph-svg path { opacity: 1; stroke-width: 2.65; }
   .graph-row:hover .graph-svg path.hidden-branch { opacity: 0.12; stroke-width: 2.2; }