Bluesky profile link points at the current handle

## Summary

`SOCIALS` links to `https://bsky.app/profile/alunduil.bsky.social`,
a handle that no longer resolves. The account is now `alunduil.com`.

## Motivation

Found while verifying Bridgy backfeed for #201. Both
`com.atproto.identity.resolveHandle` and `app.bsky.actor.getProfile`
reject `alunduil.bsky.social`; `app.bsky.actor.searchActors` returns
the account as `alunduil.com` (`did:plc:urcrp6xgybniubantb6asetr`).
Every post page and the footer carry the dead link today.

Link checking will not catch this. bsky.app is a single-page app that
answers any `/profile/` path with a 200 and resolves the
handle client-side, so lychee sees a healthy URL in both the offline
pre-commit tier and the weekly full crawl.

## Scope

- Point `src/constants.ts:32` at
`https://bsky.app/profile/alunduil.com`.

Considered and rejected: linking the DID
(`https://bsky.app/profile/did:plc:urcrp6xgybniubantb6asetr`), which
survives any future handle change but is unreadable in a status bar
and on hover. A handle change is rare enough to fix by hand.

## Acceptance criteria

- [ ] `SOCIALS` links to a Bluesky profile URL that resolves.
- [ ] No other reference to the old handle remains (one today, in
`src/constants.ts`).

## Additional context

The link carries no `rel="me"` — only the GitHub entry does
(`src/constants.ts:22`) — so IndieAuth and the webmention.io sign-in
from #148 are unaffected.