Fix: clarify README app entry note

[ishaan-jaff]

Summary

The README implied App.svelte is always the only file contributors need to edit, which is unclear for changes touching other files. This updates the note to describe App.svelte as the main app code location instead.

Repro

On main, run:

python3 - <<'PY'
from pathlib import Path
readme = Path('README.md').read_text()
if '](' in readme and 'contains the main app code.' in readme:
    print('PASS: README clearly describes App.svelte.')
else:
    print('FAIL: README still says App.svelte is all you need to edit.')
    raise SystemExit(1)
PY

It fails because the README says App.svelte "is all you need to edit."

Evidence

Tests

• python3 scripts/verify-readme.py — passed
• npm run build — not run: npm is not installed in this cloud image (--: line 1: npm: command not found)