Fix review comments: remove io.NopCloser wrapper, add HTTP timeout, fix typos

[Copilot]

Addresses PR review feedback on DownloadImage and DownloadThumbnail methods.

Changes

• HTTP timeout: Added 30-second default timeout via newHTTPClient() to prevent indefinite hangs
• Test fix: Removed io.NopCloser wrapper from failingBody tests—was silently preventing close error testing
• Typo fix: Corrected "thums" → "thumb" in thumbnail filename suffix and variable names
• Test cleanup: Consolidated duplicate image helpers into makeImageBytes(w, h, format, r, g, b), removed incomplete TestDownloadImage_CloseErrorJoined

Example

// Before: close errors never triggered because io.NopCloser provides no-op Close()
Body: io.NopCloser(b)

// After: failingBody.Close() error properly propagates
Body: b

// Before: no timeout, could hang indefinitely
fetch.New(fetch.WithHTTPClient(&http.Client{}))

// After: 30s timeout by default
var newHTTPClient = func() *http.Client {
    return &http.Client{Timeout: 30 * time.Second}
}

---

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