fix: show heading caret markers only while the editor is focused

[Azganoth]

Summary

Opening a heading-first document no longer shows its caret marker before the editor receives focus. Focus and blur update the marker without moving the selection or changing Markdown; heading hover indicators and folding remain available.

Related Issue

Closes #647

Verification

• pnpm exec vitest run src/features/editor/plugins/markerPresentation.test.ts src/features/editor/plugins/headingFold.test.ts src/features/editor/utils/headingOutline.test.ts — focus changes preserve selection, content, and undo history, and heading navigation and folding retain their behavior.

• pnpm check:frontend passed its formatting, type-checking, lint, and production-build stages. pnpm exec vitest run --coverage --maxWorkers=2 passed the full frontend suite and coverage floors.

• pnpm exec tsc -p e2e/desktop/tsconfig.json --noEmit, pnpm build:e2e:desktop, and pnpm test:e2e:desktop:run -- --scenario heading-outline passed on Windows, covering fresh opens, focus and blur, same-document reloads, outline navigation, hover markers, and folding.

• pnpm exec vitest run src/features/editor/tests/mathProjection.test.ts --maxWorkers=2 passed. Layout and table-escape assertions use a deterministic fast-render clock, while the separate costly-preview test verifies deferred rendering.

• CI passed frontend checks, backend checks, the Rust dependency audit, and the full Windows desktop E2E suite for e500222744100c26a521c498752a7ad33b46aeda.

Not verified: desktop behavior on macOS or Linux.