feat: multi-project workspaces and project window groups (#448)

Workspaces open several local project folders in one IDE session (File › New / Open / Manage Workspace…, or Attach when opening a project, which creates a workspace automatically for a plain project window). Member folders are the Project view roots and take part in indexing, Find in Files, Git, language servers, run configurations and builds; agent tasks choose which member they work in. Window › Merge All Project Windows groups project windows into one window with project tabs (switch, reorder, close, move to a new window), restored on reopen.

Closes #241
Closes #198
Closes #286

src/workbench/WindowGroupTabs.module.css

@@ -0,0 +1,62 @@
+.bar {
+  display: flex;
+  flex: none;
+  align-items: center;
+  gap: var(--island-gap);
+  padding: 0 var(--island-gap) var(--island-gap);
+  min-width: 0;
+}
+.tabs {
+  display: flex;
+  flex: 1;
+  gap: 4px;
+  overflow-x: auto;
+  min-width: 0;
+}
+.item {
+  display: flex;
+  align-items: center;
+  flex: 0 1 220px;
+  min-width: 100px;
+  border-radius: var(--radius-small);
+  color: var(--fg-muted);
+}
+.item:hover {
+  background: var(--bg-hover);
+}
+.item {
+  touch-action: none;
+  user-select: none;
+}
+.dragging {
+  opacity: 0.55;
+}
+.selected {
+  background: var(--bg-island);
+  color: var(--fg);
+  box-shadow: inset 0 -2px var(--accent);
+}
+.tab {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  flex: 1;
+  min-width: 0;
+  height: 30px;
+  padding: 0 10px;
+  border: 0;
+  color: inherit;
+  background: transparent;
+  font: inherit;
+  cursor: pointer;
+}
+.tab span {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.tab:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: -2px;
+  border-radius: var(--radius-small);
+}