Turn the root README into a proper project overview: what the app
does today, the stack, how to run it locally and the current status,
instead of the short Spanish setup notes it had before. Replace the
generic SvelteKit scaffold README in frontend/ with a short one that
actually describes this client.
Remove Go doc comments, TS/Svelte scaffold comments and explanatory
notes left over from the initial scaffold, plus a few leftover Spanish
notes in the compose file, SQL scripts and env template. Also drops
frontend/src/lib/index.ts, which only held the SvelteKit placeholder
comment and was never imported.