fix(desktop): preserve glyphs at transcript clip edges

[liuxiaocs7]

Summary

Fix assistant text whose leading glyph pixels are clipped by transcript paint containment. Keep content-visibility: auto, but add a 2px overflow-clip-margin so Chromium can paint glyph ink that extends fractionally past the flush-left boundary without changing the message box geometry.

Extend WideAssistantProse to assert that the assistant boundary keeps content visibility, receives the clip margin, and still reaches the existing full-width right edge.

This is an alternative implementation to #4904. It adjusts the paint clip rather than padding and margins, so transcript layout remains unchanged.

Fixes #4898

Verification

• npm --workspace @maka/desktop run smoke:storybook — passed (310 stories, 337 theme renders), including product-shell-official-appshell--wide-assistant-prose
• npm --workspace @maka/desktop run build-storybook — passed
• npm --workspace @maka/desktop run typecheck — passed
• npx biome check apps/desktop/src/renderer/styles/chat-message.css apps/desktop/stories/app-shell.stories.tsx — passed
• git diff --check — passed
• Chromium pixel comparison: the fixed render is pixel-identical to the uncontained reference (pixel_diff=0); zero-margin containment removed 13 pixels within the first 1–4px of the line
• Oversized-turn check: all 96 transcript boundaries remain present and 14 offscreen boundaries remain skipped, preserving the rendering bound

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex diagnosed the paint-containment regression, implemented the CSS fix and Storybook contract, ran verification, and drafted this pull request. The commit includes the required Generated-by: Codex trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

Thanks. This is the one to take for #4898.

Head e03e5bc. overflow-clip-margin on the existing [data-maka-transcript-boundary] rule is the smallest fix that keeps content-visibility: auto and does not move the text. WideAssistantProse still asserts the flush right edge, and CI is green.

#4904 aims at the same bug with padding-inline plus a negative margin. That changes layout to dodge a paint clip. Storybook smoke already failed it: product-shell-official-appshell--wide-assistant-prose got expected 8 to be less than or equal to 1. Closing that PR as superseded.

One P3: the new play asserts overflowClipMargin >= 2, not pixels. Fine here; jsdom cannot see the compositor clip. The pixel_diff against the uncontained reference is what shows 2px is enough.

I am not approving yet. Someone still needs to look at product-shell-official-appshell--partial-history-notice on this head (CJK first glyphs on each line) before merge.

Reviewed with Grok.

[Astro-Han]

Looked at PartialHistoryNotice: CJK first glyphs on each line are intact. Merging this over #4904.

Head e03e5bc.