Emdash source with visual editor image upload fix

Fixes:
1. media.ts: wrap placeholder generation in try-catch
2. toolbar.ts: check r.ok, display error message in popover

packages/core/tests/unit/plugins/page-contribution-sandbox.test.ts

@@ -0,0 +1,71 @@
+import { describe, it, expect } from "vitest";
+
+/**
+ * Tests for the sandbox boundary enforcement of page contribution hooks.
+ *
+ * page:metadata is sandbox-safe.
+ * page:fragments is trusted-only but valid in manifests (enforcement happens
+ * at runtime via capability checks and at bundle time via CLI warnings).
+ *
+ * The enforcement happens at multiple layers:
+ * 1. Manifest schema: HOOK_NAMES includes both page:metadata and page:fragments
+ * 2. Capability enforcement: page:fragments requires page:inject capability
+ * 3. Bundle CLI: warns when page:fragments is declared in a sandbox-targeted plugin
+ * 4. Fragment collector: never invokes sandboxed plugins for page:fragments
+ */
+
+describe("page contribution sandbox boundary", () => {
+	describe("manifest schema validation", () => {
+		it("should accept page:metadata in manifests", async () => {
+			const { pluginManifestSchema } = await import("../../../src/plugins/manifest-schema.js");
+
+			const manifest = {
+				id: "test-plugin",
+				version: "1.0.0",
+				capabilities: [],
+				allowedHosts: [],
+				storage: {},
+				hooks: [{ name: "page:metadata" }],
+				routes: [],
+				admin: { pages: [], widgets: [] },
+			};
+
+			const result = pluginManifestSchema.safeParse(manifest);
+			expect(result.success).toBe(true);
+		});
+
+		it("should accept page:fragments in manifests (enforcement is at runtime)", async () => {
+			const { pluginManifestSchema } = await import("../../../src/plugins/manifest-schema.js");
+
+			const manifest = {
+				id: "test-plugin",
+				version: "1.0.0",
+				capabilities: [],
+				allowedHosts: [],
+				storage: {},
+				hooks: [{ name: "page:fragments" }],
+				routes: [],
+				admin: { pages: [], widgets: [] },
+			};
+
+			// Manifest validation accepts page:fragments — trusted-only enforcement
+			// happens via capability checks (requires page:inject) and the bundle CLI
+			// warns when this hook is used in a sandbox-targeted plugin.
+			const result = pluginManifestSchema.safeParse(manifest);
+			expect(result.success).toBe(true);
+		});
+	});
+
+	describe("fragment collector defense-in-depth", () => {
+		it("resolveFragments only processes contributions it receives", async () => {
+			// The fragment collector in page/fragments.ts is a pure function that
+			// processes whatever contributions are passed to it. The defense-in-depth
+			// is that the runtime never passes sandboxed plugin contributions to it.
+			// This test verifies the pure function works correctly.
+			const { resolveFragments } = await import("../../../src/page/fragments.js");
+
+			const result = resolveFragments([], "head");
+			expect(result).toEqual([]);
+		});
+	});
+});