docs(integration): record Square auth lifecycle merge

.project-docs/10-decisions/adr-004-square-auth-lifecycle-boundary.md

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+# ADR-004: Works Square Owns The Desktop Authentication Lifecycle Boundary
+
+## Status
+
+Accepted and implemented on 2026-08-19.
+
+## Context
+
+The desktop client obtained tokens from Works Square but refreshed them directly against the custom identity service with an embedded OAuth client secret. That mixed issuer/client boundary made rotation dependent on two independently configured clients and could turn an otherwise valid persisted session into `invalid_grant`, returning the user to the login page hours later.
+
+## Decision
+
+- Renderer sends authentication operations only to Electron Main.
+- Electron Main owns access/refresh tokens, encrypted persistence, refresh rotation, terminal failure cleanup, and the seven-day inactivity policy.
+- Desktop login, mobile login, refresh, and logout use fixed Works Square `/api/auth/*` endpoints.
+- Works Square owns the confidential upstream OAuth client configuration and proxies the lifecycle to the identity service.
+- The desktop bundle must not contain an OAuth client secret or call the custom identity service directly.
+
+## Consequences
+
+- The matching Works Square server endpoints must be deployed before this client is released.
+- OAuth credential rotation or upstream endpoint changes are server-side configuration changes rather than desktop releases.
+- A terminal refresh `400` or `401` still fails closed and clears the local session; transient failures preserve the established retry behavior.
+
+## Evidence
+
+- Source commit: `dc776ff`
+- Integration merge: `f52c2c8`
+- Feature verification: 2,190 client tests passed, typecheck passed, Vite build passed, and lint reported no errors.