feat(dashboard_auth): support confidential clients (client_secret) in self-hosted OIDC (#55344)

The self-hosted OIDC dashboard provider was public-client + PKCE only, with
two `# TODO(confidential-client)` seams. Authentik and Keycloak commonly
default a new OIDC client to *confidential*, whose token endpoint rejects an
unauthenticated exchange (`invalid_client`) — so a self-hoster who accepts
their IDP's default could not complete dashboard login without manually
flipping the client to public.

Add optional confidential-client support:

- New optional `client_secret` (env `HERMES_DASHBOARD_OIDC_CLIENT_SECRET`,
  or `dashboard.oauth.self_hosted.client_secret`; env-wins-config, empty
  treated as unset). It is a credential, so docs steer operators to the
  `.env` file; config.yaml is supported only for precedence symmetry.
- `_token_endpoint_auth()` selects `client_secret_basic` (HTTP Basic header)
  vs `client_secret_post` (form body) from the IDP's advertised
  `token_endpoint_auth_methods_supported`, defaulting to basic (the OIDC
  default) when absent. Applied to complete_login, refresh_session, and
  revoke_session (RFC 7009 §2.1).
- PKCE is sent in BOTH modes — the secret is client authentication layered
  on top, never a replacement (OAuth 2.1 / RFC 9700 keep PKCE mandatory).
- Basic header url-encodes client_id/secret before base64 per RFC 6749
  §2.3.1, so reserved chars (`:`, `@`, space) round-trip correctly.

Non-breaking: with no secret configured the provider is a pure public PKCE
client, byte-identical to prior behaviour (no Authorization header, no
client_secret in the body). The secret is never logged — register() reports
only a `confidential=<bool>` flag.

Tests: 16 new cases covering basic/post selection, default-when-absent,
public-unchanged contract, PKCE-preserved, reserved-char url-encoding,
blank-secret-is-public, refresh + revoke auth, no-secret-in-logs, and
env/config register wiring. Full dashboard-auth suite (nous provider,
middleware, gate, cookies, WS, 401-reauth, status endpoint) — 396 tests —
green, proving no existing auth path regressed.

cli-config.yaml.example

@@ -1276,3 +1276,33 @@ updates:
 #   # default — works on Fly.io out of the box).
 #   #
 #   #   public_url: "https://example.com/hermes"
+#
+# -----------------------------------------------------------------------------
+# Self-hosted OIDC dashboard auth (generic OpenID Connect — Authentik,
+# Keycloak, Zitadel, Authelia, Auth0, Okta, Google, …). Use this INSTEAD of the
+# nous block above when gating the dashboard with your own identity provider.
+# Each setting can be overridden by an environment variable:
+#
+#   dashboard.oauth.self_hosted.issuer         <-  HERMES_DASHBOARD_OIDC_ISSUER
+#   dashboard.oauth.self_hosted.client_id      <-  HERMES_DASHBOARD_OIDC_CLIENT_ID
+#   dashboard.oauth.self_hosted.scopes         <-  HERMES_DASHBOARD_OIDC_SCOPES
+#   dashboard.oauth.self_hosted.client_secret  <-  HERMES_DASHBOARD_OIDC_CLIENT_SECRET
+#
+# dashboard:
+#   oauth:
+#     provider: self-hosted
+#     self_hosted:
+#       issuer: "https://auth.example.com/application/o/hermes/"  # required
+#       client_id: "hermes-dashboard"                             # required
+#       scopes: "openid profile email"                            # optional
+#
+#       # OPTIONAL — set ONLY if your IDP registered the client as
+#       # *confidential* (Authentik / Keycloak often default to this). When
+#       # set, Hermes authenticates the client at the token endpoint
+#       # (client_secret_basic or client_secret_post, auto-selected from the
+#       # IDP's discovery doc) IN ADDITION to PKCE. Leave unset for a public
+#       # (PKCE-only) client — the common case.
+#       #
+#       # This is a CREDENTIAL: prefer setting HERMES_DASHBOARD_OIDC_CLIENT_SECRET
+#       # in ~/.hermes/.env over putting it here in config.yaml.
+#       #   client_secret: ""