Fix generative preview capture to return image bytes

[kylymo]

Plan 001: Return image bytes when capturing a generative preview

Executor: follow every step and verification gate. Modify only listed files. Stop and report if a STOP condition occurs.

Status

• Priority: P1
• Effort: S
• Risk: LOW
• Depends on: none
• Category: bug
• Planned at: commit c16bc4f, 2026-07-16

Why this matters

getCaptureImageFile fetches an image, but passes the Response object itself to File instead of its body bytes. Capture/download consumers can therefore receive a file whose contents are not the fetched PNG.

Current state

• app/utils/generativePreview.ts:27-40, getCaptureImageFile, validates the hash and calls fetch(imageDataPayload.image).
• Line 40 constructs new File([res], 'image.png', ...); res is a Response, not a Blob or ArrayBuffer.
• The function is a focused utility and has no visible regression test beside the existing utility test files.

Commands and expected results

Purpose	Command	Expected result
Tests	pnpm run test -- app/utils/generativePreview.spec.ts	New capture tests pass
Lint	pnpm run lint -- app/utils/generativePreview.ts	exit 0
Full tests	pnpm run test	all tests pass

Scope

In scope:

• app/utils/generativePreview.ts
• A colocated test file for capture behavior

Out of scope:

• Image-provider routing and hash generation semantics
• UI changes to capture controls

Steps

Step 1: Consume the response body

Use await response.blob() (or an equivalent byte-oriented body method) and preserve the response failure behavior by checking response.ok or mapping failures to the existing error. Keep the existing hash mismatch guard.

Verify: Add a test with a mocked successful Response and assert the returned File has the expected byte size/type.

Step 2: Cover failed fetches and mismatched hashes

Add regression cases for a mismatched hash and an unsuccessful response. Keep error messages actionable and avoid logging image contents.

Verify: pnpm run test -- app/utils/generativePreview.spec.ts → all cases pass.

Test plan

• Valid matching hash returns a PNG file containing the response body.
• Hash mismatch rejects before fetch.
• Non-OK response rejects or follows the chosen documented error contract.

Done criteria

• File contains fetched bytes, not a Response object
• Regression tests pass
• Lint passes
• Only in-scope files changed

STOP conditions

• Current response/file API behavior differs materially from the evidence.
• A test requires browser-only APIs unavailable under the repository's Vitest setup without an approved test-environment change.

Maintenance notes

Keep this utility's network error behavior stable for callers unless tests demonstrate that callers already distinguish error types.

Issue body

Bug Description

getCaptureImageFile passes a fetched Response object directly to File, so generated preview capture may not contain the downloaded image bytes.

Evidence

• app/utils/generativePreview.ts:39-40
• const res = (await fetch(...)) as any
• new File([res], 'image.png', { type: 'image/png' })

Expected Behavior

The returned File contains the response body bytes and has the expected image type.

Acceptance Criteria

• Add regression tests for successful capture, hash mismatch, and failed response.
• Use response.blob() or equivalent byte extraction.
• pnpm run test and pnpm run lint pass.

Environment

• Repository: chaotic-art/app
• Baseline: c16bc4f
• Node: package declares 24.x; audit host had Node 22.22.1
• pnpm was unavailable during audit