feat(servers): export server list as mcp.json download (#1346) (#1351)

* feat(servers): export server list as mcp.json download (#1346)

Adds an Export button to the Servers screen, next to "Add Servers".
Clicking it downloads the current server list as a canonical mcp.json
file (2-space indent, same shape serializeStore writes), so the user
can hand it off, sync it to another machine, or symlink into Claude
Desktop / Cursor / Cline.

Wiring:
- ServerListControls: new Export Button (variant=default), disabled
  when serverCount === 0 (nothing to download).
- ServerListScreen / InspectorView: thread `onExport` / `onServerExport`
  through.
- App.tsx: onServerExport runs serverEntriesToMcpConfig on the in-memory
  servers list, JSON.stringify with 2-space indent, and triggers a Blob
  download via a temporary anchor (appended to body for Firefox
  compatibility, then removed; object URL revoked).

Tests:
- ServerListControls unit tests cover button-count (toggle hidden vs
  shown, Export always present), disabled-on-empty, click fires onExport.
- ServerListControls.stories grows play functions for both stories —
  WithServers asserts Export is enabled + click fires the spy in real
  Chromium, WithoutServers asserts Export is disabled.
- Existing screen + view tests / stories updated for the new prop.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(servers): address PR #1351 review (extract helpers + exception safety + guard)

Substantive items from the review:

1. Serialization format parity (review #1): added
   `serializeMcpConfig(entries)` to core/mcp/serverList.ts — a
   browser-safe helper that runs `serverEntriesToMcpConfig` then
   stringifies with 2-space indent. Single source of truth for export
   formatting; can't drift from serializeStore's on-disk shape.

2. Exception safety on the anchor cleanup (review #2) + a direct test
   for the App.tsx download wiring (review #6): extracted
   `downloadJsonFile(filename, json)` to clients/web/src/lib/
   downloadFile.ts. removeChild + revokeObjectURL run in a `finally`
   so a thrown click() doesn't leak the DOM node or the object URL.
   Unit-tested under happy-dom with URL.createObjectURL mocked
   (happy-dom doesn't ship it).

3. Empty-list guard (review #5): App.tsx onServerExport bails when
   `servers.length === 0`. The button's disabled prop already prevents
   the UI path, but the handler is now locally correct against any
   future programmatic caller (storybook, keyboard shortcut, etc.).

Acknowledged but not changed:
- Tooltip on the disabled Export button (review #3) — Mantine's
  disabled-button + Tooltip wart wants a span wrapper + custom events
  config. Optional per the reviewer; deferred.
- Repeated exports → `mcp (1).json` browser suffix (review #4) — a
  deliberate trade-off since `mcp.json` is the conventional name a user
  would expect to symlink into Claude Desktop / Cursor / etc.

Tests: 1647 unit (+6) + 410 integration + 306 storybook = 2363 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/web/src/lib/downloadFile.ts

@@ -0,0 +1,32 @@
+/**
+ * Browser-side helpers for triggering file downloads from in-memory content.
+ *
+ * Centralized so the temp-anchor incantation (`appendChild` for Firefox,
+ * try/finally to keep cleanup bulletproof, `revokeObjectURL` to release the
+ * object URL) lives in one place — and so the wiring is unit-testable
+ * under happy-dom without dragging React along.
+ */
+
+/**
+ * Download an in-memory string as a JSON file. Uses a temporary anchor
+ * element to trigger the browser's save dialog. The append-to-body step is
+ * for older Firefox versions that wouldn't fire `click()` on a detached
+ * anchor; modern browsers don't require it but it stays as the safe path.
+ *
+ * The cleanup (removeChild + revokeObjectURL) runs in a `finally` so even
+ * a thrown `click()` doesn't leak the DOM node or the object URL.
+ */
+export function downloadJsonFile(filename: string, json: string): void {
+  const blob = new Blob([json], { type: "application/json" });
+  const url = URL.createObjectURL(blob);
+  const anchor = document.createElement("a");
+  anchor.href = url;
+  anchor.download = filename;
+  document.body.appendChild(anchor);
+  try {
+    anchor.click();
+  } finally {
+    document.body.removeChild(anchor);
+    URL.revokeObjectURL(url);
+  }
+}