[codex] structure Electron theme source errors - #3294

Reviewed the changes to apps/desktop/src/electron/ElectronTheme.ts and apps/desktop/src/electron/ElectronTheme.test.ts against the Effect service conventions. No violations found.

Key points verified:

• Imports: Effect modules use namespace subpath imports (import * as Schema from "effect/Schema"); @t3tools/contracts correctly uses a named import for the whole package.
• File order: Canonical order preserved — imports, error/predicate declarations, Context.Service tag with inline interface, make, then layer (Layer.succeed).
• Error modeling: ElectronThemeSetSourceError uses Schema.TaggedErrorClass with structured attributes. The variable source field provides domain context, and cause preserves the immediate underlying failure (required, since every construction wraps a real failure). Schema.Defect() is used only for cause; the tag plus source identify the failure structurally, which is the allowed pattern.
• Message derivation: message is derived solely from the stable source attribute and does not stringify cause (the test even asserts notInclude(error.message, cause.message)).
• Predicate: isElectronThemeSetSourceError is exported directly as Schema.is(...), not a private constant wrapped by a redundant function.
• No redundant discriminator: A single precise error tag is used without an added singleton operation/reason/kind field.
• Tests: The behavior change (theme set failures now produce a typed error) is covered by a focused test that uses a test layer rather than mocking core business logic.

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Agent Credits: 23 credits