refactor: trim custom-endpoint comments and share synthetic provider const

Review pass: shorten verbose doc comments across the custom-endpoint code,
merge duplicate imports, share one synthetic provider entry between the keys
section and setup dialog, collapse the duplicated connect-editor block, revert
an unrelated settings-route drive-by.

src/core/auth-keys.ts

@@ -123,16 +123,11 @@ export const AUTH_PROVIDER_KEYS: readonly AuthProviderInfo[] = [
 ] as const;
 
 /**
- * THE SINGLE CUSTOM ENDPOINT's identity (custom-endpoint wave): a
- * user-configured OpenAI-compatible server registered into pi-ai at runtime
- * (server/custom-provider.ts). Deliberately NOT part of AUTH_PROVIDER_KEYS —
- * that table is test-pinned to pi-ai's BUILT-IN providers
- * (tests/auth-provider-keys.test.ts) and every row is env-var-keyed; the
- * custom endpoint is keyed by a user-supplied BASE URL instead, and its API
- * key rides this one reserved env var (OPTIONAL — keyless local servers such
- * as Ollama / LM Studio are allowed). The Custom endpoint Settings section is
- * its one home; the key is stored in auth.json under `envVar` like any other
- * provider key (masked, env-reflected, no restart).
+ * THE single custom endpoint's identity: a user-configured OpenAI-compatible
+ * server registered into pi-ai at runtime (server/custom-provider.ts). NOT part
+ * of AUTH_PROVIDER_KEYS (that table is test-pinned to pi-ai's built-ins and is
+ * env-var-keyed); this one is keyed by a user-supplied base URL, and its key
+ * rides this reserved env var (optional — keyless local servers allowed).
  */
 export const CUSTOM_PROVIDER = {
 	id: "custom",