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.

frontend/pnpm-lock.yaml

@@ -8,6 +8,9 @@ importers:
 
   .:
     dependencies:
+      construct-style-sheets-polyfill:
+        specifier: ^3.1.0
+        version: 3.1.0
       epubjs:
         specifier: ^0.3.93
         version: 0.3.93
@@ -1166,6 +1169,9 @@ packages:
     resolution: {integrity: sha512-L6BuueHTRuJHQvQVc6YXYZRtN5vJUtOdCTLn0tRYYV5azfbAFcPghB5zEE40mVrV6w7slMTqUfkDomutIK14fw==}
     engines: {node: '>=20'}
 
+  construct-style-sheets-polyfill@3.1.0:
+    resolution: {integrity: sha512-HBLKP0chz8BAY6rBdzda11c3wAZeCZ+kIG4weVC2NM3AXzxx09nhe8t0SQNdloAvg5GLuHwq/0SPOOSPvtCcKw==}
+
   cookie@0.6.0:
     resolution: {integrity: sha512-U71cyTamuh1CRNCfpGY6to28lxvNwPG4Guz/EVjgf3Jmzv0vlDp1atT9eS5dDjMYHucpHbWns6Lwf3BKz6svdw==}
     engines: {node: '>= 0.6'}
@@ -3346,6 +3352,8 @@ snapshots:
       semver: 7.7.4
       uint8array-extras: 1.5.0
 
+  construct-style-sheets-polyfill@3.1.0: {}
+
   cookie@0.6.0: {}
 
   core-js@3.48.0: {}