fix: boot splash never freezes with arms missing

[tuckcode]

A Tauri/WebKit window on macOS often reports document.hidden at first paint. The splash then set --boot-motion-state: paused while the grow-in arms use animation-fill-mode: both from opacity 0. If visibilitychange never fires when the window is shown, those two arms stay invisible and the breathe pulse stays frozen.

This change keeps motion running until the page has actually been visible once. After that, a later hide can still pause. Paused and prefers-reduced-motion: reduce both drop the grow animations so the full settled logo stays on screen.

Rebased onto main after #150. Handoff note is docs/plans/handoffs/2026-10-11-0815-cursor-grok-4-6-boot-splash-arms.md only — docs/HANDOFF.md is not edited.

Mac rebuild check: launch the app; the two arms grow in and the logo pulses; with Reduce Motion on, the full logo shows still.

What changed

• index.html no longer pauses grow-in just because the first paint is hidden.
• visibilitychange to visible still resumes motion.
• BootSplash.css shows the settled mark when paused or when motion is reduced.

Completion

• QA: unit tests for hidden-at-boot, visibility resume, paused CSS, and reduced motion. Native Mac launch not run in this environment.
• Tests/coverage: pnpm exec vitest run src/index.bootMotion.test.ts src/components/BootSplash.test.tsx — 10 passed. Frontend lane (pnpm lint && pnpm typecheck && pnpm build && coverage shards && pnpm test:mcp) passed: 88.44% lines (threshold 70%), 106 mcp tests. sh .husky/pre-push itself skipped app checks because the branch was already on the remote (empty @{push}..HEAD). Rust skipped (no src-tauri/ change).
• Localization: none — English only (C18).
• PostHog: no event needed because this restores existing splash motion; it does not add a user action.
• Refactoring: none needed.
• ADRs: none.
• Docs: session note in docs/plans/handoffs/2026-10-11-0815-cursor-grok-4-6-boot-splash-arms.md. docs/HANDOFF.md left unchanged (docs: stop PRs from conflicting in HANDOFF.md #150).
• Demo vault dirt: none.
• Codacy: CLI not installed on this machine. No new Critical/High expected from CSS/HTML visibility handling.

  

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[codacy-production]

Pull Request Overview

The required automated scenarios are covered except native Mac/Tauri launch validation, so the platform-specific acceptance criterion remains unverified. No code, security, complexity, or Codacy issues were reported.

About this PR

• Validate the splash during a native Mac/Tauri launch to confirm that the arms grow and the logo pulses as required.

Test suggestions

• Hidden at first paint keeps boot motion running
• Visibility change to visible resumes motion
• A later hide pauses motion and showing the page resumes it
• Paused CSS disables grow-in animations so all arms remain visible
• Reduced-motion CSS disables animations while retaining the settled logo
• Native Mac/Tauri launch confirms arms grow and logo pulses

Prompt proposal for missing tests

Consider implementing these tests if applicable:
1. Native Mac/Tauri launch confirms arms grow and logo pulses

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