feat: add session document viewers and PDF controls

[wibus-wee]

Related issue

None — internal same-repository change.

Problem / pressure

Session links could preview PDFs, but their reading controls were limited. DOCX,
XLSX, and PPTX fell back to the generic binary-file notice; CSV/TSV had only a
source-text view. Loading format engines with the file panel would also impose
work on sessions that never open these files.

Summary

• Add PDF page thumbnails, direct page entry, rotation, fit/percentage zoom, and
  expandable text search while retaining the 64 KiB local range transport and
  8-megapixel canvas cap.
• Add read-only DOCX, XLSX, and PPTX previews. The static Office entry waits for
  an active panel and an idle browser period, bounds reads to 25 MiB, and loads
  only the selected format engine. Parsers use workers; inactive previews do not
  request an engine.
• Add a worker-parsed, two-axis-virtualized CSV/TSV table with search and zoom.
  Keep the existing editable Source tab and terminate its worker when hidden.
• Preserve shared file actions, add synthetic Storybook fixtures and Playwright
  acceptance coverage, and update the bilingual Spec and implementation notes.

Visual explanation

flowchart TD
    A[Session file snapshot] --> B{File format}
    B -->|PDF| C[PDF.js page viewer]
    C --> D[64 KiB local ranges + bounded canvases]
    B -->|DOCX / XLSX / PPTX| E[Static Office preview entry]
    E -->|active + browser idle| F[Bounded read, then selected engine + worker]
    B -->|CSV / TSV| G{Preview or Source}
    G -->|Preview active + idle| H[Worker parse/search + virtual table]
    G -->|Source| I[Existing text editor]
    C --> J[Existing file actions]
    F --> J
    H --> J

Loading

Before / after

Before	After
PDF preview had basic paging.	Page thumbnails, direct page entry, rotation, zoom modes, and search.
Modern Office files showed a binary notice.	DOCX pages, XLSX worksheets, and PPTX slides open read-only on demand.
CSV/TSV opened as source text.	Virtual table by default, with Source still available for editing.

Test plan

• Passed: pnpm format, pnpm lint, pnpm lint:i18n,
  pnpm check:public-boundary, and pnpm --filter @lody/components typecheck.
• Passed: 8 targeted Vitest tests and 9 Playwright Storybook acceptance tests,
  including narrow PDF controls and inactive Office/CSV loading checks.
• Passed: pnpm --filter @lody/components build-storybook; verified the Office
  engines are split into on-demand resources.
• pnpm check is blocked at the pre-existing nested
  packages/acp-extension-claude checkout because its own node_modules and
  @tsconfig/node22 are missing in this worktree. Component checks above pass.
• pnpm docs check reports six existing links into uninitialized ACP extension
  submodules; no changed-document link errors remain.
• Real Electron lody-resource:// rendering remains unverified here.

Context handoff

Internal same-repository branch; the external contribution handoff does not apply.