fix(preview): relative iframe src for subpath deployment (bd-0pm7a) (#226)

* fix(preview): relative iframe src for subpath deployment (bd-0pm7a)

Q2DebugIframe and Q2PreviewIframe hard-coded `src="/q2-*.html"`, which
is root-absolute and bypasses Vite's `base: './'`. Locally and in
Playwright the SPA is served at the host root so the two coincide;
under a subpath deployment the iframe requested `https://host/q2-*.html`
and got the platform's 404 / SPA shell, blanking the preview. Dropping
the leading slash resolves the iframe against the parent doc's
pathname, matching how the rest of the build references its assets.
Hash-only routing keeps the parent's pathname stable across client
navigation, so the relative form is correct at every route.

* sync beads (close bd-0pm7a)

Author: shikokuchuo