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/th.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 85.333 512 341.333"><path fill="#FFF" d="M0 85.334h512V426.66H0z"/><path fill="#0052B4" d="M0 194.056h512v123.882H0z"/><g fill="#D80027"><path d="M0 85.334h512v54.522H0zM0 372.143h512v54.522H0z"/></g></svg>