fix: require PKCE S256 on public OAuth clients

[nanookclaw]

Problem

authorizeSubmit and autoAuthorize pass code_challenge through to issueCode without checking whether it is present. When a client omits the challenge, the Token handler's PKCE verification is silently skipped because the stored codeChallenge is empty. For public clients (no client secret), PKCE is the only proof-of-identity during code exchange.

Fix

Both authorization paths now reject requests where code_challenge is absent or code_challenge_method is not S256, returning 400 before any code is issued. This moves PKCE from opt-in to mandatory.

Closes #31

[vercel]

@nanookclaw is attempting to deploy a commit to the freedisch's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Freedisch]

Thanks for your contribution
Have you run some local tests to see if it breaks the auth ?

[nanookclaw]

Ran the local Go test suite from the module root after re-checking this branch:

cd havril && go test ./...

Result: all packages pass; the repo currently has no Go test files, so this verifies compile/package integrity rather than exercising an auth integration test.

I also manually re-read the OAuth flow touched by this PR:

• authorizeSubmit now rejects missing/non-S256 PKCE before validating the token or issuing a code.
• autoAuthorize applies the same check before silent authorization.
• Existing Claude/MCP clients that send code_challenge_method=S256 continue through the same issueCode → Token exchange path.

So this should not break the current Claude auth flow; it only rejects clients that omit PKCE or use a non-S256 method.