feat: language picker with flags, paging and follow mode, translation run in the floating window

Language picker with search and SVG flags, real pagination with locate, one global machine translation run shown next to uploads.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/public/flags/cz.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 513 342"><path fill="#11457e" d="M0 0h513v342H0z"/><path fill="#d7141a" d="M513 171v171H0l256.5-171z"/><path fill="#FFF" d="M513 0v171H256.5L0 0z"/></svg>