feat(layout): re-probe the layout when the workspace's plugin states change

The backend now activates layout providers PER WORKSPACE (the workspace's
per-workspace enablement state picks the ONE active layout plugin), so
toggling a layout plugin in the workspace plugins modal changes which tree
serves the workspace. The modal's toggle now refreshes the layout so the
newly-enabled provider's tree (or the no-provider degrade) lands without a
reload:

- WorkspaceLayoutPort gains refresh(workspaceId) — the modal calls it
  after every state write (an attempted write re-probes either way: the
  probe is the idempotent catch-up; a refused write changed nothing, so it
  re-syncs the same tree).
- App.svelte wires the port to the layout store's new
  onWorkspacePluginsChanged(workspaceId) — a guarded re-probe that only
  fires for the ACTIVE workspace (another workspace's toggle is picked up
  by its own probe on switch, exactly like the other §2.6 triggers).

The per-workspace tree resolution itself is backend-side; the marker
renderer, the probe triggers, and the WS  push filtering are
unchanged.

Verification: typecheck (svelte-check 0 errors) / test (1528) / check
/ build all green.

2 files changed, 59 insertions, 1 deletions

diff --git a/src/features/plugins/ui/WorkspacePluginModal.svelte b/src/features/plugins/ui/WorkspacePluginModal.svelte
index d1bb27a..0ba7cd6 100644
--- a/src/features/plugins/ui/WorkspacePluginModal.svelte
+++ b/src/features/plugins/ui/WorkspacePluginModal.svelte
@@ -38,6 +38,11 @@
 
   async function toggle(pluginId: string, checked: boolean): Promise<void> {
     await plugins.setWorkspaceState(workspaceId, pluginId, checked ? "enabled" : "disabled");
+    // The per-workspace activation rule (layout): toggling a plugin may have
+    // changed which layout plugin serves THIS workspace — re-probe so the
+    // newly-enabled provider's tree (or the no-provider degrade) renders
+    // without a reload. The store ignores non-active workspaces.
+    layout?.refresh(workspaceId);
   }
 
   // ── The reset affordance (plan 04 §4.4) ─────────────────────────────────
diff --git a/src/features/plugins/ui/WorkspacePluginModal.test.ts b/src/features/plugins/ui/WorkspacePluginModal.test.ts
index 6bc7179..4c75ab3 100644
--- a/src/features/plugins/ui/WorkspacePluginModal.test.ts
+++ b/src/features/plugins/ui/WorkspacePluginModal.test.ts
@@ -51,8 +51,9 @@ async function renderModal(http: PluginsHttp = fakeHttp()) {
 function fakeLayoutPort(
   providerActive = true,
   reset: WorkspaceLayoutPort["reset"] = vi.fn(async () => null),
+  refresh: WorkspaceLayoutPort["refresh"] = vi.fn(),
 ): WorkspaceLayoutPort {
-  return { providerActive, reset };
+  return { providerActive, reset, refresh };
 }
 
 describe("WorkspacePluginModal", () => {
@@ -117,6 +118,58 @@ describe("WorkspacePluginModal", () => {
     expect(fake.setWorkspaceState).toHaveBeenCalledWith("ws", "mcp", "disabled");
   });
 
+  it("a successful toggle calls the layout port's refresh (the per-workspace activation may change the served tree)", async () => {
+    const user = userEvent.setup();
+    const refresh = vi.fn();
+    const store = createPluginsStore(fakeHttp());
+    await store.openWorkspace("ws");
+    render(WorkspacePluginModal, {
+      props: {
+        workspaceId: "ws",
+        workspaceTitle: "My Workspace",
+        plugins: store,
+        layout: fakeLayoutPort(
+          true,
+          vi.fn(async () => null),
+          refresh,
+        ),
+        onClose: vi.fn(),
+      },
+    });
+    await user.click(screen.getByRole("checkbox", { name: /Enable mcp/i }));
+    expect(refresh).toHaveBeenCalledWith("ws");
+  });
+
+  it("a REFUSED toggle (state write fails) still calls refresh — the probe is the idempotent catch-up", async () => {
+    const user = userEvent.setup();
+    const refresh = vi.fn();
+    const fake = fakeHttp({
+      setWorkspaceState: async () => ({
+        ok: false,
+        error: "Plugin is not loaded — load it globally first",
+      }),
+    });
+    const store = createPluginsStore(fake);
+    await store.openWorkspace("ws");
+    render(WorkspacePluginModal, {
+      props: {
+        workspaceId: "ws",
+        workspaceTitle: "My Workspace",
+        plugins: store,
+        layout: fakeLayoutPort(
+          true,
+          vi.fn(async () => null),
+          refresh,
+        ),
+        onClose: vi.fn(),
+      },
+    });
+    await user.click(screen.getByRole("checkbox", { name: /Enable mcp/i }));
+    // The attempted state change re-probes either way (a refused write changed
+    // nothing, so the probe re-syncs the same tree — harmless and consistent).
+    expect(refresh).toHaveBeenCalledWith("ws");
+  });
+
   it("shows the template hint for the default workspace", async () => {
     const store = createPluginsStore(fakeHttp());
     await store.openWorkspace("default");