Split view, personal modes, budget warnings, server move script

- Up to three chats side by side (#/s/a+b+c), "Open beside" in the sidebar menu
- File manager tabs of a chat stored per user on the server (/sessions/{id}/tabs)
- Personal modes for every user (Settings → My modes, /account/modes)
- Budget meters (total, provider, images, opencode Go windows) with push warnings
  for admins at the warning share and 100 %, shown in the status overlay
- scripts/move-server.sh: move to another Docker host with bind mounts (Unraid);
  scripts/smoke-test.sh

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/styles.css

@@ -3725,3 +3725,48 @@ span.chat-tab > button {
 .compacting-card.auto {
   border-color: color-mix(in srgb, var(--accent) 60%, var(--bg3));
 }
+.status-meter .budget-bar {
+  width: 70px;
+  flex: none;
+}
+.status-meter .pct {
+  width: 38px;
+  text-align: right;
+  font-variant-numeric: tabular-nums;
+}
+
+/* --- split view (chats side by side) ------------------------------------------- */
+.split-view {
+  flex: 1;
+  min-height: 0;
+  display: flex;
+}
+.split-pane {
+  flex: 1;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+}
+.split-pane + .split-pane {
+  border-left: 1px solid var(--bg3);
+}
+.split-pane .topbar {
+  flex-wrap: nowrap;
+}
+.split-pane .topbar .grow {
+  min-width: 90px;
+}
+.split-pane .topbar h1 {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+/* Narrow columns: the context chip moves out (the cost chip opens the usage overlay). */
+.split-pane .topbar span.chip.hide-mobile {
+  display: none;
+}
+@media (max-width: 900px) {
+  .split-pane + .split-pane {
+    display: none;
+  }
+}