Keep usage logins visible and explain unavailable quota (#5876)

* fix(usage): keep existing logins visible and explain unavailable quota

Credential discovery must preserve accounts whose login exists even when it has expired. Keep vendor failures on the discovered account and isolate discovery failures without inventing provider cards.

* test(usage): align credential route tests with account discovery

packages/plugin/src/server/usage.ts

@@ -38,13 +38,28 @@ export interface UsageDetail {
   tone?: UsageWindow["tone"];
 }
 
-export interface UsageReport {
-  status: "available" | "unavailable" | "error";
-  planLabel?: string;
-  windows: UsageWindow[];
-  balances?: UsageBalance[];
-  details?: UsageDetail[];
-  error?: string;
+export type UsageProblem =
+  | { kind: "expired"; expiresAt: string; refreshedBy?: string }
+  | { kind: "rejected"; status: number; refreshedBy?: string }
+  | { kind: "no_quota"; detail: string };
+
+export type UsageReport =
+  | {
+      status: "available";
+      planLabel?: string;
+      windows: UsageWindow[];
+      balances?: UsageBalance[];
+      details?: UsageDetail[];
+    }
+  | { status: "unavailable"; problem: UsageProblem }
+  | { status: "error"; error: string };
+
+export interface UsageAccount {
+  /** Stable across token rotation; [A-Za-z0-9._-]{1,128}. Never a credential or raw email. */
+  key: string;
+  label?: string;
+  /** Store locator, opaque to the daemon. */
+  input: JsonValue;
 }
 
 export interface UsageSourceRegistration {
@@ -52,10 +67,10 @@ export interface UsageSourceRegistration {
   label: string;
   icon?: string;
   input: ZodType;
-  /** Stable account identity, resolved without fetching usage. */
-  identify(input: unknown): Promise<{ key: string; label?: string } | null>;
+  /** Every account whose login exists on this machine. Empty when none. */
+  discover(): Promise<UsageAccount[]>;
+  /** Re-reads the login store; never writes it. */
   fetch(input: unknown): Promise<UsageReport>;
-  discover(): Promise<JsonValue[]>;
 }
 
 export function windowFromUsedPct(input: {
@@ -122,11 +137,6 @@ export function hashAccountKey(value: string): string {
   return createHash("sha256").update(value).digest("hex");
 }
 
-export function unavailableUsage(): UsageReport {
-  return {
-    status: "unavailable",
-    windows: [],
-    balances: [],
-    details: [],
-  };
+export function unavailable(problem: UsageProblem): UsageReport {
+  return { status: "unavailable", problem };
 }