Fix I-1: landing-page edits lost when activePageId goes dangling

buildSavePayload matched the active page by page.id === activePageId,
but activePageId defaults to the hardcoded 'home' and load() never
updated it. Deleting the original Home page (its replacement gets id
page_<ts>), reloading, editing, and saving would then match no page --
the live edit only reached the legacy top-level craft_state, while the
authoritative pages[]/pages_craft_state[0] for index.html fell back to
stale state.

Belt-and-suspenders fix:
- buildSavePayload now falls back to pages[0] as the effective active
  page when a real page is active but activePageId matches nothing.
- useWhpApi's load() now points activePageId at the freshly restored
  first page via a new PageContext setActivePageIdDirect setter
  (bookkeeping only -- no re-serialize/deserialize), so activePageId
  stays valid after every load.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/hooks/useWhpApi.save.test.ts

@@ -87,4 +87,33 @@ describe('buildSavePayload', () => {
     expect(payload.header_craft_state).toBe('STALE_HEADER');
     expect(payload.footer_craft_state).toBe('STALE_FOOTER');
   });
+
+  test('I-1: activePageId matches no page (dangling, e.g. original Home deleted then reload reset it to "home") -- live serialize still lands in pages_craft_state[0] / index.html, not lost', () => {
+    // pages[0] has id 'page_x' (the replacement landing page after the
+    // original 'home' was deleted); activePageId is stuck at the stale
+    // default 'home', which matches no entry in `pages`.
+    const pageX: PageData = { id: 'page_x', name: 'Home', slug: 'index', craftState: 'STORED_X' };
+    const p2: PageData = { id: 'p2', name: 'About', slug: 'about', craftState: 'STORED_ABOUT_2' };
+
+    const payload = buildSavePayload({
+      siteId: 1,
+      siteName: 'Test Site',
+      liveCraftState: 'LIVE_EDIT',
+      pages: [pageX, p2],
+      headerPage,
+      footerPage,
+      activePageId: 'home',
+      isEditingHeader: false,
+      isEditingFooter: false,
+    });
+
+    // The live edit must land in pages_craft_state[0] (index.html slot),
+    // not be silently dropped to only the legacy top-level fields.
+    expect(payload.pages_craft_state[0].craftState).toBe('LIVE_EDIT');
+    expect(payload.pages[0].filename).toBe('index.html');
+    // Top-level fields (legacy) should also reflect the live edit.
+    expect(payload.craft_state).toBe('LIVE_EDIT');
+    // The other page is untouched.
+    expect(payload.pages_craft_state.find((p) => p.id === 'p2')?.craftState).toBe('STORED_ABOUT_2');
+  });
 });