feat: add hierarchical block selection

[Azganoth]

Summary

• Add a Leafdown-owned directional block selection for root and nested blocks, retaining exact cross-parent endpoints so adjacent list items are never absorbed through hidden wrapper promotion.
• Add local pointer gutters with first-line-aligned handles, Shift-range interaction, drag preparation, accessible announcements, and source-projection-safe mapping and history.
• Reuse the existing context popup with rendered block bounds, block-aware actions, neutral selection presentation, and focused desktop coverage for narrow layouts and nested structures.

Related Issue

Closes #478

Verification

• pnpm exec vitest run src/features/editor/plugins/blockSelection.test.ts src/features/editor/plugins/blockSelectionInteraction.test.ts src/features/editor/plugins/blockSelectionProjection.test.ts src/features/editor/plugins/contextPopup.test.ts src/features/editor/commands/formatting/blocks.test.ts src/features/editor/tests/editorPresentation.test.ts verifies exact cross-list ranges, replacement, mapping, source projection, pointer interaction, popup integration, formatting, and presentation.
• pnpm check:frontend verifies formatting, TypeScript, Oxlint, the production build, and the complete frontend coverage suite.
• pnpm build:e2e:desktop builds the assembled desktop test binary.
• pnpm test:e2e:desktop:run -- --scenario block-selection verifies nested gutter geometry, handle visibility and expansion, selection stability, popup behavior, focus, and exact cross-list presentation in the Windows WebView.

Not verified: the full desktop E2E suite or a separate manual interactive Tauri pass.

Notes

Cross-parent ranges use exact selectable-block endpoints and ProseMirror open structural slices instead of promoting endpoints to list wrappers. This intentionally supersedes the original lowest-common-ancestor promotion detail in #478 because promotion visually and operationally absorbed adjacent list items; the final behavior is recorded in the specification and decision record.