test(file-preview): give the stubbed harnesses rendersAsPage()

Three suites load panels-ui.js without constants.js and stub the globals
it uses (escapeHtml, CodemanBase). openFilePreview() now asks
rendersAsPage() from constants.js, so those stubs need it too; none of
their files is an HTML page, so it answers false.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/file-preview-detach.test.ts

@@ -40,6 +40,8 @@ function loadApp() {
     escapeHtml: (s: string) => String(s),
     // Reverse-proxy route builder from constants.js (not loaded here); identity at root.
     CodemanBase: { base: '', url: (p: string) => p },
+    // Also from constants.js: no file in this suite renders as an HTML page.
+    rendersAsPage: () => false,
     document: { getElementById: () => null, addEventListener: vi.fn() },
     window: windowStub,
     setTimeout,

test/file-preview-media.test.ts

@@ -66,6 +66,8 @@ function loadApp(media: FakeMedia[]) {
     escapeHtml: (s: string) => String(s),
     // Reverse-proxy route builder from constants.js (not loaded here); identity at root.
     CodemanBase: { base: '', url: (p: string) => p },
+    // Also from constants.js: no file in this suite renders as an HTML page.
+    rendersAsPage: () => false,
     document: { getElementById: () => null, addEventListener: vi.fn() },
     window: { addEventListener: vi.fn() },
     setTimeout,

test/spreadsheet-preview.test.ts

@@ -594,6 +594,8 @@ describe('file-preview overlay spreadsheet wiring', () => {
       console,
       escapeHtml: (value: unknown) => String(value ?? ''),
       CodemanBase: { base: '', url: (p: string) => p },
+      // From constants.js (not loaded here): a workbook never renders as an HTML page.
+      rendersAsPage: () => false,
       fetch: fetchMock,
       setTimeout,
       clearTimeout,