fix(resource-renderers): show MB when an image size rounds to 1024 KB - #1096

Problem

formatByteLength (the size shown in the image viewer's info line and in the image diff) picks KB for anything under 1 MiB, then rounds. Sizes in the last half-kilobyte before 1 MiB round up to 1024, so they are shown as 1024 KB instead of moving to MB:

formatByteLength(t, 1024 * 1024 - 1) // main: "1024 KB"   expected: "1.0 MB"

A 1,048,575-byte image reads 1024 KB while a 1,048,576-byte one reads 1.0 MB.

Approach

The KB figure is formatted first, and KB is used only while that rounded figure is below 1024; otherwise the existing MB branch formats it. Every other size, and the 1-decimal/0-decimal rule, is unchanged.

Changes

• imageState.ts: formatByteLength checks the rounded KB figure before choosing KB.
• index.test.ts: one size just under 1 MiB reads 1.0 MB, one that rounds to 1023 stays 1023 KB, and exactly 1 MiB stays 1.0 MB. The first case fails on main (1024 KB) and passes with the change.

Screenshots

Not applicable: the visible effect is the size text above (1.0 MB instead of 1024 KB).

Related issues

None found.

Checklist

• Fast gates pass: bun run lint, bun run typecheck, bun run test. I ran biome check (2.5.0) on the two changed files and the new cases against imageState.ts with bun test (the full index.test.ts needs the locales package installed): they pass, and the first one fails without the fix. I did not run the full workspace gates.
• E2E suite passes for app-affecting changes. Not run.
• Before/after screenshots are included for frontend changes, or marked not applicable
• Relevant SPEC.md / top-level specs updated to reflect any boundary, contract, or behavior change (no contract change)
• I have read the Contributing guide and agree to the Code of Conduct

This change was prepared with AI assistance; I reviewed it and ran the tests above locally.