account-pane: switch preflight to GET-based discovery (zero-mutation)

[melvincarvalho]

Why

The account-pane (#12 / PR #13) preflights with an unauthenticated `PUT /idp/credentials` to detect whether the dedicated handler exists (only added in JSS 0.0.165). On a JSS <0.0.165 server with a world-writable ACL on `/idp/credentials`, the wildcard could create an empty file at that path before the pane bails.

WAC normally rejects unauth PUT before the wildcard runs (#13 conversation), so this is theoretical, but it's residual risk worth eliminating.

Proposed fix

Two-part change:

Server (JSS)

Extend `handleCredentialsInfo` (`src/idp/credentials.js`) to enumerate supported methods:

```diff
export function handleCredentialsInfo(request, reply, issuer) {
return {
endpoint: `${issuer}/idp/credentials`,

• method: 'POST',

• methods: ['GET', 'POST', 'PUT'], // PUT only on 0.0.165+
  description: '...',
  ...
  };
  }
  ```

Bump JSS to 0.0.166. The new `methods` field declares feature support without touching any state.

Client (this repo)

Replace the PUT-probe with a GET to `/idp/credentials`:

```js
async function preflight() {
try {
var res = await fetch(endpoint)
if (res.status !== 200) return false
var info = await res.json().catch(() => null)
return !!(info && Array.isArray(info.methods) && info.methods.includes('PUT'))
} catch (e) { return false }
}
```

GET is idempotent — zero mutation risk regardless of WAC config or JSS version.

Acceptance

• JSS PR adds `methods` array to GET /idp/credentials response
• JSS publishes 0.0.166
• account-pane preflight switches to GET-based discovery
• PR account-pane: phase 1 — change-password form (#12) #13's residual-risk comment in account-pane.js can be removed

Out of scope

• Pure-OPTIONS-based discovery (less reliable, JSS's CORS plugin returns the same Allow regardless of dedicated routes)
• Removing the preflight entirely — even with this fix, a server that omits the `methods` field should fail closed