refactor(ui): split chat-header.css into single-surface files (#546 PR2)

[Astro-Han]

Summary

Phase A relocation pass (zero visual change) for chat-header.css, which held six unrelated surfaces. Each surface now converges into one correctly-named file, per the #546 PR2 plan.

Why

Refs #546. The post-#520 audit found chat-header.css (972 lines) mixing 6 roles: toast / palette / help+confirm / error+fake-banner / hero / chat-header chrome. You can't polish a surface whose styles are scattered, so this PR relocates first (one surface, one file), with no visual change, governed by the converge-contract pattern established in #584.

Scope

Split out of chat-header.css (+ chat-message.css):

New file	Content moved
toast.css	.maka-toast-* (viewport, card, variants, icon/copy/action/close, enter/exit keyframes)
palette.css	.maka-palette-* + the shared .maka-shortcut-* kbd primitive (densest user is the palette)
help.css	.maka-help-* + .maka-confirm-modal
error.css	.maka-error-* + .maka-fake-backend-banner
hero.css	.maka-hero base + all of chat-message.css (it was 100% empty-state hero, 0 message selectors)

Plus one cross-file move:

• The composer streaming top-sweep (.maka-composer-inner[data-streaming="true"]::before + @keyframes maka-processing-sweep) moves from chat-header.css into composer.css, next to the card chrome (rest / focus) it decorates.

What remains in chat-header.css: the header bar itself (.maka-chat-header, status cluster, alert/status pills) + the chat shell layout (.maka-chat-shell / chatViewport / chatContent / chat-jump-bottom). The chat shell is a distinct surface not named in the PR2 scope, so it's left for a later PR rather than moved to an unspecified target.

chat-message.css is deleted — it was 100% hero content. PR4 will repopulate it when the chat bubble is extracted from TSX into CSS.

Contract follow-ups (filename pins that pointed at chat-header.css now point at palette.css):

• radius-nesting-contract — .maka-palette-input-wrap label
• renderer-important-audit-contract — .maka-palette-input-wrap input:focus allowlist entry
• menu-highlight-recipe-contract — doc comment

Not included: any polish, token, or selector-value change. No TSX touched.

Verification

Relocate-only proof. The multiset of CSS rules across the original (chat-header.css + chat-message.css + composer.css) is byte-identical to the new layout — 204 rules, 0 lost / 0 added, all 5 @keyframes preserved (incl. maka-processing-sweep). Selector prelude + declaration body matched per rule.

Contracts green. npm run -w @maka/desktop test → 2134 pass / 0 fail, including:

• renderer-tailwind-compile-contract (compiles the full styles.css import tree)
• composer-container-single-source-contract (streaming ::before carries [data-streaming], so it's excluded from the rest-state single-source count — unaffected by the move)
• all converge contracts (typography, radius, spacing, control-height, box-shadow, letter-spacing, font-weight, z-index, focus-ring)
• the three filename-pinned contracts above, updated

npm run -w @maka/desktop typecheck clean.

Screenshots: not attached. This is a pure CSS relocation with proven rule-equivalence, so there is no visual delta to capture. The repo's own guidance is that the screenshot harness fixes viewport/seed and state-dependent regressions slip through it; the rule-equivalence proof + converge contracts are stronger evidence of zero visual change here. Dual-theme screenshot verification can be added on request.

User-facing impact

None. Zero visual change by construction.

Reviewer notes

• The composer streaming snippet is the only rule whose cascade position shifts (it was in chat-header.css, imported before composer.css; now it's inside composer.css). Its selector .maka-composer-inner[data-streaming="true"]::before has no competing rule today (the old tool-output.css ::before { content: none } reset was already removed), so moving it later in the cascade only strengthens it — no visual change.
• chat-message.css → hero.css shows as a git rename (72% similarity); the remainder is the .maka-hero base rule moved in from chat-header.css.

Checklist

• Scope matches the PR title and excludes unrelated changes
• Verification lists commands/results
• User-facing impact: none (zero visual change)
• Risk: none beyond a pure relocate; rollback is a single revert
• UI changes: screenshots N/A — rule-equivalence proof + contracts (justified above)