refactor(oidc): remove non-standard oidc=1 marker param

[Summpot]

Why

The OIDC flow used a bespoke oidc=1 query parameter as a marker to let the login SPA recognize an OIDC authorization. That marker is non-standard: in the standard OIDC authorization-code flow with PKCE, the authorization endpoint redirects an unauthenticated user to the login page carrying only the standard authorization parameters (client_id, redirect_uri, scope, state, code_challenge, code_challenge_method, nonce). The presence of those required parameters is already sufficient to detect an OIDC flow, so oidc=1 added no value.

Approach

Remove the oidc=1 marker everywhere:

• Worker (crates/beacon-worker/src/wasm/oidc.rs): stop appending oidc=1 in the two unauthenticated/expired redirect branches.
• Rust server (crates/beacon/src/handlers/oidc.rs): stop appending oidc=1 in login_redirect.
• Frontend (src/lib/minecraft-flow.ts): drop oidc from oidcSearchSchema and drop the params.oidc === '1' check from isOidcFlow.
• Login/register links: stop forwarding the removed oidc param.
• src/router.tsx: updated the comment (the string-preserving parseSearch/stringifySearch fix from fix(ui): keep OIDC query params as strings to fix invalid_value error #2 remains necessary, since OIDC params are strings).

Verification

• cargo build -p beacon-worker --target wasm32-unknown-unknown passes (only pre-existing unused-alias warning).
• The beacon server crate could not be checked locally due to a pre-existing OpenSSL/MSVC environment dependency issue unrelated to this change; the change there is a pure deletion of an append_pair line.

Notes

No compatibility concern: the mod builds the authorize URL itself and never reads the oidc query param; the removed param was only read by this frontend's schema. The string-preserving search parsing already merged in #2 is kept.