Use Fluent.js and convert all existing reader strings

[dstillman]

https://github.com/zotero/reader/pull/142#issuecomment-2408363159

[mrtcode]

As discussed in https://github.com/zotero/reader/pull/142#issuecomment-2408855682, use Fluent.js for all builds except Zotero client, because it already supports Fluent out of the box.

Convert all existing strings.

Note that PDF.js also uses Fluent.js and likely includes it in its source, even though we don't use localized strings from PDF.js. Check how much space it adds to the PDF.js build and consider excluding it if necessary.