feat(files): an HTML file opens as the page — Files panel, Chat preview, File Explorer

Clicking an .html/.htm file now renders it instead of showing its source or
downloading it:
- Changes/Files panel: a whole HTML file opens on Preview (Raw = the editor).
- Chat file preview: a workspace page brings its relative files along.
- File Explorer: double-click / Enter opens a viewer; Download stays on its
  header and right-click menu.

The page runs in the chat artifact's sandbox (iframe srcdoc, scripts, never
allow-same-origin). Its relative stylesheets (and their url()s), scripts,
images, media and icons are read by the dashboard through the existing
download routes and inlined as <style>/data: URLs (lib/htmlInline.ts: never
above the folder's root, ≤ 60 files / 30 MB) — the login cookie is
SameSite=Strict, so a frame could not fetch them itself, and no new route
serves files without it.

client/src/components/run/__tests__/ChatFilePreview.test.tsx

@@ -242,7 +242,9 @@ describe("the artifact view", () => {
       ((await screen.findByTestId("artifact-frame")) as HTMLIFrameElement).getAttribute("srcdoc")
     ).toContain("two");
     fireEvent.click(versions.querySelector("button")!);
-    expect(screen.getByTestId("artifact-frame").getAttribute("srcdoc")).toContain("<p>one</p>");
+    await vi.waitFor(() =>
+      expect(screen.getByTestId("artifact-frame").getAttribute("srcdoc")).toContain("<p>one</p>")
+    );
     cleanup();
   });