docs: add /qtapp/ Qt6-wasm runner

[MMesch]

Ships a self-contained in-browser runner for Qt6-wasm packages from emscripten-forge at `emscripten-forge.org/qtapp/`. Given a `.tar.bz2` URL (or a locally-picked file), it fetches, unpacks, and boots the Qt app entirely client-side using `@emscripten-forge/untarjs`.

• `docs/qtapp/{index.html, runner.js}`: the runner itself, ~350 LOC total. Loaded as static assets: MkDocs copies non-markdown files in `docs/` verbatim, so no `mkdocs.yml` change is needed. Fetches unpack.wasm and emscripten-forge/untarjs .
• `docs/usage/qtapp.md`: new docs page describing usage, deep-links (`?pkg=`, `?fullscreen=1`), and known limitations. Auto-picked up by the MkDocs nav.
• `docs/index.md`: new bullet under Usage linking to the docs page.

[MMesch]

noticed that it currently doesn't work on Firefox. Investigating.

[MMesch]

works now. JSPI needs to be enabled. I added JSPI detection and a better error message for this now.