[codex] structure Electron theme source errors

[juliusmarminge]

Summary

• expose Electron theme-source assignment failures as a schema-backed error
• retain the requested theme source and exact Electron cause
• add focused coverage without expanding into currently overlapping theme read/subscription consumers

Verification

• vp test apps/desktop/src/electron/ElectronTheme.test.ts
• vp check (0 errors; 20 existing warnings)
• vp run typecheck

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Note

Low Risk
Small, localized Electron theme wrapper change; setTheme IPC may now surface failures that previously could throw uncaught, but scope is non-critical UI theming.

Overview
ElectronTheme.setSource no longer assumes native themeSource assignment always succeeds. It now runs the Electron setter inside Effect.try and fails with a schema-backed ElectronThemeSetSourceError that carries the requested DesktopTheme (source) and the underlying defect (cause), plus a stable user-facing message that names the source but not the raw cause.

The service type and a Schema.is guard are exported so callers (e.g. IPC setTheme) can narrow failures. Tests extend the Electron mock to throw on set and assert error shape, source, cause, and message content.

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Note

Structure Electron theme source errors into a typed ElectronThemeSetSourceError

• Adds ElectronThemeSetSourceError, a Schema.TaggedErrorClass carrying the requested source and original cause, replacing raw thrown errors from nativeTheme.themeSource assignment.
• Updates ElectronTheme.setSource return type from Effect<void> to Effect<void, ElectronThemeSetSourceError>, making failures explicit to callers.
• Adds isElectronThemeSetSourceError type guard for consumers to narrow the error type.
• Behavioral Change: errors previously propagated as unhandled throws now surface as typed failing effects.

^{Macroscope summarized c1ea123.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/desktop-electron-theme-errors

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[macroscopeapp]

Approvability

Verdict: Approved

This PR adds structured error handling for the Electron theme source setting operation. The core behavior is unchanged - it only improves how errors are captured and typed. The changes are self-contained with good test coverage.

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