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/features/app/WorkspaceDialog.module.css

@@ -0,0 +1,68 @@
+.body {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+  padding: 20px;
+}
+.field {
+  display: grid;
+  grid-template-columns: 80px 1fr;
+  align-items: center;
+  gap: 12px;
+}
+.row {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  min-width: 0;
+}
+.row > input {
+  flex: 1;
+  min-width: 0;
+}
+.heading {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  font-weight: 600;
+}
+.projects {
+  min-height: 130px;
+  max-height: 260px;
+  overflow: auto;
+  border: 1px solid var(--border);
+  border-radius: var(--radius-small);
+}
+.project {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 10px 12px;
+}
+.project + .project {
+  border-top: 1px solid var(--border);
+}
+.path {
+  display: flex;
+  flex: 1;
+  min-width: 0;
+  flex-direction: column;
+  gap: 3px;
+}
+.path span {
+  color: var(--fg-muted);
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.hint {
+  color: var(--fg-muted);
+  margin: 0;
+  line-height: 1.5;
+}
+.projects > .hint {
+  padding: 16px;
+}
+.error {
+  color: var(--danger);
+}