docs: bring the Integrate page and version references up to date

docs.html was missing everything from this session's work:
- The disconnectedCallback/crypto-worker-teardown gotcha (found via the
  BitRoad integration) -- now a callout right where the element is first
  mounted, since that's the step where it actually bites.
- Talking to the element directly (getPublicKey/signEvent/nip44Encrypt/
  nip44Decrypt as instance methods) as an alternative to window.nostr,
  with BitRoad's adapter as the worked example.
- claimSigner() failing gracefully (returns false, doesn't throw) when a
  NIP-07 extension has locked window.nostr down -- was previously
  undocumented entirely; this is the exact bug just fixed.
- Theming via CSS custom properties, with the full property list and
  BitRoad's mapping as a real example.

Also fixed two stale "(v0.3)" references to docs/spec.md in README.md and
CONTRIBUTING.md -- the spec was bumped to v0.4 earlier this session for the
rollback-detection changes but these two mentions were missed.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

README.md

@@ -7,7 +7,7 @@ password that unlocks a portable Nostr identity, plus an independent 12-word
 recovery phrase. Encrypted account material lives on public Nostr relays —
 no account server, no password-reset email, no database.
 
-This repo implements the protocol from `docs/spec.md` (v0.3) as a modular
+This repo implements the protocol from `docs/spec.md` (v0.4) as a modular
 system any static site can embed for login and Nostr signing.
 
 ## Layout