feat: jspod landing page + AGPL-3.0 relicense (#11)

* chore: relicense to AGPL-3.0-only

jspod's runtime dependency JSS is AGPL-3.0-only. A MIT label on
this wrapper was a license-chain conflict — when a project links
to AGPL code, the combined work is effectively AGPL regardless of
the wrapper's stated license. Aligning jspod's declared license
with the practical reality.

- LICENSE: full AGPL-3.0 text (replaces MIT)
- package.json: "license": "AGPL-3.0-only"
- README: badge + footer updated, noting that the change matches JSS

* feat: ship jspod-flavored landing page (overwrites pod-data/index.html)

Stopgap until JSS gets a downstream-customization hook: on every
start, after JSS has finished pod init, jspod copies its
welcome.html into pod-data/index.html. JSS serves that file at /
exactly as it would its own default.

What the new landing page does over JSS's default:
- "me / me" credentials visible inline (no need to scroll the
  terminal banner)
- Pointer to a Solid app (Pilot) for the actual sign-in flow
- Directory tiles for /profile/, /public/, /private/, /inbox/,
  /settings/ so the new user can explore their pod immediately
- Same HEAD-adaptive Sign in / Sign up logic as JSS's template
  (probes /idp/register; 200 → both, 403 → Sign in only, 404 → hide)
- Same cream + white-card visual language as the data browser

Implementation:
- New welcome.html in the package (added to files whitelist)
- copyFileSync from <pkg>/welcome.html to <root>/index.html after
  waitForReady resolves true. Best-effort; failures are silent and
  the user falls back to whatever JSS already wrote.
- One shared readiness promise drives both the overwrite and the
  auto-open path so we don't poll twice.

The overwrite is unconditional on every start — user customizations
to pod-data/index.html will be lost. This is intentional for now;
a real architectural fix needs upstream JSS support and will be
filed as a separate issue.

The welcome page leaves pod-data/index.html.acl alone (JSS wrote it
on first init with a public-read ACL).

* chore: bump version to 0.0.13

Author: melvincarvalho