Fix reduced-motion gap in AutofillCollapse chevron

[WilfordGrimley]

Summary

• WCAG 2.2 non-contrast audit of the unified /editor + /print surface (target size, focus-visible, reduced-motion, APCA advisory) turned up one genuinely trivial, safely-fixable item: AutofillCollapse's disclosure chevron rotated via a plain inline style={{ transition: "all 0.25s 0s" }}, the only animated surface in the codebase with no prefers-reduced-motion guard (every other animation - PagePreview.tsx's loading sweep, printingTags/cardPanel.tsx, WhatsThatWords.tsx - already has one).
• Lifted the transition into a tiny styled.h5 (ChevronIcon) carrying the same transition: all 0.25s 0s plus @media (prefers-reduced-motion: reduce) { transition: none; }, matching this codebase's own established pattern. Same className, same DOM shape, only the style prop moved.
• Everything else found in the audit (target-size offenders, a systemic low-contrast focus-ring issue, and the APCA advisory table) is reported separately for the board - see this PR's own follow-up report, not shipped here, since none of it qualified as a trivial in-place fix (see "Why nothing else is in this PR" below).

Why nothing else is in this PR

• Target size (page-preview-slot-menu-cue/-flip, 9-17px measured across viewports, not the "26px" assumed in the audit brief): the 26x26 CSS size is real (confirmed via toHaveCSS in DisplayLeftRailFidelity.spec.ts), but it lives inside the sheet's own print-scale transform: scale(), so the actual on-screen/pointer-hit footprint shrinks with it at every practical viewport. Not a padding fix - a structural scale-compensation change, and the 26x26 value is itself spec-locked (SPEC-editor-polish.md EPcue).
• .fbtoggle Front/Back toggle (55x23/51x23, confirmed failing the SC 2.5.8 spacing exception too): its padding: 2px 12px/font-size: 11px are binding SPEC-editor-polish.md §D.1 (EP6) values - changing them needs an owner spec decision, not a unilateral pixel edit.
• Bootstrap's stock .btn-close (21x21, used in every Modal/Offcanvas header): a real gap, but reaches far beyond /editor and /print (~15+ modals sitewide) - out of this task's scope to touch blind.
• The focus-ring contrast finding (Bootstrap's default box-shadow focus ring computes to ~1.2-1.5:1 against this dark theme's own surfaces, well under the 3:1 non-text guidance) is systemic to every .btn/.form-control sitewide, not an /editor-or-/print-local fix.

Test plan

• npx tsc --noEmit - clean
• npx prettier@2.7.1 --check src/components/AutofillCollapse.tsx - clean
• npm test (jest) - 65 suites / 573 tests pass, unchanged
• npx playwright test tests/DisplayLeftRailFidelity.spec.ts tests/GridSelectorModal.spec.ts - 34/34 pass (both mount AutofillCollapse)
• Ad-hoc Playwright verification (not committed): page.emulateMedia({ reducedMotion: "reduce" }) -> chevron transitionDuration computed as 0s; reducedMotion: "no-preference" -> unchanged 0.25s

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