chore: vendor foliate-js

foliate-js has no npm release and no build step; upstream recommends a git
submodule. Copy it instead: this repo has no submodules and already vendors
pdf.js the same way under static/pdfjs, and a submodule would pull in 231
files / 13 MB of which 191 files / 12 MB is a bundled pdf.js build that
Chitai does not use.

Only the import closure reachable from view.js is vendored — 15 files, 584K.
pdf.js is replaced by a stub that throws: view.js reaches it through a
static-string dynamic import inside makeBook, which Rollup resolves at build
time even though Chitai serves PDFs from static/pdfjs/web/viewer.html, and
upstream's version opens with a bare `import '@pdfjs/pdf.min.mjs'` that does
not resolve here.

scripts/vendor-foliate.sh pins the commit and makes the next update a one-line
change. fixed-layout.js needs construct-style-sheets-polyfill, so add it.

.gitattributes

@@ -1,2 +1,5 @@
 # Mark pdfjs as vendored code
-frontend/static/pdfjs/** linguist-vendored
+frontend/static/pdfjs/** linguist-vendored
+
+# Mark foliate-js as vendored code
+frontend/src/lib/vendor/** linguist-vendored