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]>

CONTRIBUTING.md

@@ -51,7 +51,7 @@ reachable from most contributor sandboxes either.
 
 ## The spec is canonical
 
-`docs/spec.md` (currently v0.3) is the source of truth for the protocol
+`docs/spec.md` (currently v0.4) is the source of truth for the protocol
 itself — wire formats, KDF parameters, capsule structure, derivation paths,
 threat model. Code comments reference it by section (`§11.2`, `§17.4`, …).