feat(auth): real OIDC login + remove agents page

Replaces the dev-bypass-only AuthProvider with a real backend-mediated
OIDC client. Backend ships the OIDC plumbing in
Dialectic.Backend@2463129; this SPA drives it.

Auth flow:
  1. On mount: GET /api/auth/oidc/status (is OIDC configured?) + GET
     /api/auth/me (who am I — reads the dialectic_session HttpOnly
     cookie if present).
  2. Anon + click 'login' → window.location = /api/auth/oidc/start
     (backend redirects to IdP authorize URL with PKCE + state).
  3. IdP returns to backend /api/auth/oidc/callback → backend redirects
     here to /oidc/callback#oidc_ticket=<base64url>.
  4. OidcCallbackPage POSTs the ticket to /api/auth/oidc/exchange →
     backend Set-Cookie's the session JWT → refresh() AuthProvider →
     navigate to /.
  5. Header shows user.name + 'logout' button. Logout → POST
     /api/auth/logout (cookie cleared by backend) → user state cleared.

UI changes:
  - Header gets a Login button (acid primary style) when anon + OIDC
    enabled; logged-in pill (name + logout) when authenticated.
  - 'oidc not configured' message when backend reports
    /api/auth/oidc/status enabled=false (operator needs to run
    dialectic-cli config oidc first).
  - Removed /agents/:id route + AgentActivity page + nav link entirely
    (per user request; admin endpoint stays on backend for curl).
  - Removed Link to /agents/* from TopicDetail + Verdict pages.

api.ts:
  - Adds credentials:'same-origin' on every fetch so the session cookie
    rides along.
  - Drops the admin: opt + x-dialectic-admin-key header path (no more
    AgentActivity consumer).
  - Keeps dev-bypass header support gated on VITE_OIDC_DEV_BYPASS for
    dev convenience — backend's OIDC_ONLY=true env disables it server-
    side anyway.

types.ts: AgentSummary type removed.

Build delta: 178KB → 179KB / gzip 57.12 → 57.70 (oidc-client overhead
is just the AuthProvider polling code; no oidc-client-ts lib needed
since the backend drives the redirect flow).

src/types.ts

@@ -68,18 +68,3 @@ export interface Verdict {
   produced_at: string;
 }
 
-// Admin endpoint (next commit) shape — declare now so frontend compiles.
-export interface AgentSummary {
-  agent_id: string;
-  key_provisioned: boolean;
-  signups_count: number;
-  arguments_count: number;
-  verdicts_count: number;
-  recent_topics: Array<{
-    topic_id: string;
-    title: string;
-    status: TopicStatus;
-    role: Camp | 'volunteer';
-    last_action_at: string;
-  }>;
-}