refactor(ui): converge attachment thumbnail + file card onto maka radius/surface tokens

[Astro-Han]

Summary

Converge the two attachment surfaces (image thumbnail in chat-view, file card in composer + sent turns) onto Maka's radius-token + surface-alpha system. They were the last chat-area holdouts still styled with raw shadcn semantic aliases (bg-muted, border-border) and the deprecated rounded-lg. Tail of #546 PR5 (chat-message polish).

Why

Refs #546

Every other chat surface uses the --foreground-alpha-* surface fills and --radius-* tokens; these two lagged behind. The file card also had a concentric-radius defect: inner icon tile and remove button shared the outer card's 8px radius instead of stepping down a tier.

Scope

Changed:

• Thumbnail placeholder (packages/ui/src/chat-view.tsx): bg-muted → bg-[var(--foreground-alpha-6)] (matches the file card's own placeholder fill), border-border → border-[var(--border)], and drop the /60 magic alpha stacked on the 50%-ink --muted-foreground (it dipped below the contrast floor).
• Radius (chat-view.tsx + packages/ui/src/attachment-file-card.tsx): retire the deprecated rounded-lg alias → rounded-md (surface, 8px) on thumbnail + lightbox; file card's inner icon tile and remove button → rounded-sm (control, 6px) so the nested corners read concentric.
• Contract (apps/desktop/src/main/__tests__/attachment-frontend-contract.test.ts): a governance (#546) block pins the converged form so the surfaces cannot drift back to shadcn aliases / deprecated radius.

Not included: image-attachment fixture + screenshot pipeline coverage (the pending/lightbox states need a seeded fixture — no runtime behavior changes here to warrant it).

Verification

• npm run -w @maka/desktop test → 2264 pass / 0 fail (radius-governance, foreground-alpha, chat-cascade contracts, and check-console/a11y/copy all green).
• npm run -w @maka/desktop build:main + full @maka/* builds clean (tsc no type errors).
• New governance assertions execute and pass; the existing renderToStaticMarkup(ChatView) render test still passes.

User-facing impact

Effectively none visible. Confirmed by static token math (values are fixed oklch alphas / px, not render-dependent):

• File card inner corners 8px → 6px (rounded-md → rounded-sm): the only persistent visible change — a 2px tighter radius on the icon tile + remove button, only when a non-image attachment (pdf/doc/…) is present. This is the intended concentric-radius fix.
• Image thumbnail radius unchanged (rounded-lg and rounded-md both resolve to --radius-surface = 8px; alias hygiene only).
• Placeholder fill 5% → 6% foreground alpha (1% delta, not perceptible) and loading-spinner tint (transient) are the only other deltas.

No docs / changelog / migration impact.

Reviewer notes

The rounded-lg→rounded-md swaps are behavior-neutral by design — styles.css aliases both to --radius-surface. The real geometry change is rounded-md→rounded-sm on the file card's nested elements only.

Checklist

• Scope matches the PR title and excludes unrelated changes
• Verification lists commands/results
• User-facing impact noted (2px file-card inner radius; rest imperceptible/transient)
• Risk/review focus called out (only real geometry change is the concentric-radius step-down)
• UI changes: no screenshot — the sole persistent delta is a computed 2px radius on the file card's inner tile/button, derived from fixed radius tokens (8px→6px) and pinned by the computed-style contract; not render-environment dependent