fix(desktop): upgrade the chat-chrome invariant to a rendered computed-style contract #1312

Part of #1303 (surfaced by Codex review of #1308).

Context

#1308 deleted scripts/check-chat-chrome-screenshot.mjs, which inspected rendered pixels of the chat chrome to assert: the shell gutter is visibly distinct from the content card, there is no dark 1px seam at the sidebar/content boundary, and the bottom rounded corners expose shell/shadow pixels.

The surviving apps/desktop/src/main/__tests__/chat-chrome-no-gradient-contract.test.ts is a source-string contract: it reads the renderer CSS and regex-matches the first rule body for each selector. That locks the source declarations (flat --surface-canvas shell, no linear-gradient, no border-right on the darwin session panel, 12px radius + box-shadow on the floating card) but not the rendered cascade — a later @layer override, an adjacent element's shadow, a token merge, or clipping could pass the source test while the rendered UI regresses. So the rendered chat-chrome invariant the deleted pixel script guarded is not fully replaced.

Acceptance

• Upgrade the chat-chrome invariant to a rendered computed-style contract. Pattern: scripts/check-chat-marker-computed-style.mjs loads the built renderer CSS into a headless Electron window and reads getComputedStyle.
• Assert the rendered facts the pixel script used to check: shell background differs from the content card, no sidebar/content seam (no effective 1px dark border), the content card has a 12px radius and a non-empty box-shadow.
• Green in both light and dark. The existing source-string contract can stay as a fast pre-check; the rendered contract is the authoritative lock.

Notes

Per AGENTS.md, computed-style/text contracts are the repo's preferred regression lock over fixed screenshots, so this aligns the chat-chrome invariant with that convention instead of restoring the pixel matrix.