fix(mobile): Send composer images in upload wire format

[juliusmarminge]

Extracted from #3910.

Converts client-only draft image attachments to the upload wire shape for direct turn starts and queued outbox retries, with focused composer-image tests.

Validation inherited from #3910: vp check and mobile native lint passed before the split. The full typecheck is currently blocked in this workspace by unavailable mobile dependencies (expo-blur, expo-quick-actions, and @tabler/icons-react-native).

Note

Fix composer image attachments to strip client-only fields before upload

• Adds toUploadChatImageAttachments in composerImages.ts to map draft attachments to the wire format, omitting id and previewUri.
• Updates projectThreadStartTurnInput.ts and use-thread-outbox-drain.ts to apply this conversion before sending attachments.
• Behavioral Change: uploaded image payloads no longer include client-only id or previewUri fields.

^{Macroscope summarized 519da2d.}

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Note

Low Risk
Narrow serialization fix at send boundaries with a unit test; no auth or persistence changes.

Overview
Composer image drafts keep extra client fields (id, previewUri) for UI; startTurn expects UploadChatImageAttachment, which only includes type, name, mimeType, sizeBytes, and dataUrl.

This PR adds toUploadChatImageAttachments in composerImages.ts and uses it when building turn input in buildProjectThreadStartTurnInput (new threads / queued creation) and in use-thread-outbox-drain when retrying queued messages on existing threads. A unit test asserts the stripped wire shape.

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[macroscopeapp]

Approvability

Verdict: Approved

Straightforward bug fix that strips client-only fields from image attachments before sending over the wire. The change is small, well-tested, and has clear intent with limited scope.

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