feat: add Atlas native app UX overhaul

Docs/Sequence/execute-flow.md

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+# Execute Flow
+
+## Actors
+
+- User
+- AtlasApp
+- AtlasWorkerClient
+- AtlasWorkerXPC
+- AtlasPrivilegedHelper
+- AtlasStore
+
+## Sequence
+
+1. User previews a plan and confirms execution.
+2. App sends `task.execute`.
+3. Worker splits actions into privileged and non-privileged work.
+4. Worker performs non-privileged actions directly.
+5. Worker submits allowlisted privileged actions to helper when needed.
+6. Worker streams progress, warnings, and per-item results.
+7. Worker persists task result and recoverable items.
+8. App renders a result page with success, warnings, and recovery actions.