feat(exporters): real PDF/PPTX/ZIP via system Chrome + pptxgenjs + zip-lib (lazy-loaded)

[hqhq1025]

Goal

Replace the EXPORTER_NOT_READY stubs in packages/exporters/src/{pdf,pptx,zip}.ts with working tier-1 implementations. All three formats ship as real exporters; the heavy runtime deps load lazily so the Electron cold-start bundle is unchanged (PRINCIPLES §1, §6).

Files in / out

Modified

• packages/exporters/src/pdf.ts — puppeteer-core driving the user's installed Chrome
• packages/exporters/src/pptx.ts — pptxgenjs, one slide per <section> (or one for the whole doc)
• packages/exporters/src/zip.ts — zip-lib bundling index.html + README.md + optional assets
• packages/exporters/src/index.ts — isExporterReady(...) returns true for all four formats; exportArtifact dispatches with payload
• packages/exporters/package.json — adds three prod deps (see below)
• apps/desktop/src/main/exporter-ipc.ts — comment update only (channel/payload unchanged)
• apps/desktop/src/renderer/src/components/PreviewToolbar.tsx — flips dropdown items from "Coming in Phase 2" to per-format hover hints

Created

• packages/exporters/src/chrome-discovery.ts — Mac/Win/Linux Chrome resolver + CODESIGN_CHROME_PATH override
• packages/exporters/src/chrome-discovery.test.ts — 6 tests (per-OS path, override, EXPORTER_NO_CHROME)
• packages/exporters/src/pdf.test.ts — 3 tests, puppeteer-core mocked
• packages/exporters/src/pptx.test.ts — 6 tests including '你好世界' CJK round-trip with the real library
• packages/exporters/src/zip.test.ts — 3 tests, real zip-lib extract round-trip

Untouched (boundary respected)

• apps/desktop/src/main/index.ts, apps/desktop/src/renderer/src/App.tsx, apps/desktop/src/renderer/src/store.ts, apps/desktop/src/preload/index.ts

New dependencies

Package	License	Install size	Lazy-loaded?
puppeteer-core@^24.10.0	Apache-2.0	~3 MB (no Chromium download)	yes — await import('puppeteer-core') inside exportPdf
pptxgenjs@^3.12.0	MIT	~2.5 MB	yes — await import('pptxgenjs') inside exportPptx
zip-lib@^1.0.4	MIT	~80 KB	yes — await import('zip-lib') inside exportZip

All three are Apache-2.0-compatible. Lazy-load proof: grep -E "^import .* from ['\"](puppeteer-core\|pptxgenjs\|zip-lib)['\"]" packages/exporters/src/*.ts returns nothing — every reference is inside an async function via dynamic import(). Total impact stays well inside the 80 MB installer budget; no Chromium is bundled (we discover the system Chrome at runtime and throw EXPORTER_NO_CHROME with the install link if missing).

Acceptance test outcomes

$ pnpm -r typecheck     # all 10 workspaces green
$ pnpm lint             # only one pre-existing complexity warning in PasteKey.tsx (unrelated)
$ pnpm -r test          # 18/18 exporter tests pass
   ✓ src/chrome-discovery.test.ts (6 tests)
   ✓ src/pdf.test.ts (3 tests)              — puppeteer-core mocked
   ✓ src/pptx.test.ts (6 tests, 582ms)      — real pptxgenjs, CJK '你好世界' round-trip, valid PK zip header
   ✓ src/zip.test.ts (3 tests)              — real zip-lib extract round-trip

No silent fallbacks: each exporter throws CodesignError with a structured code (EXPORTER_NO_CHROME / EXPORTER_PDF_FAILED / EXPORTER_PPTX_FAILED / EXPORTER_ZIP_FAILED).

§5b checklist

• Compatible — ExporterFormat, ExportResult, IPC channel codesign:export, and the renderer's ExportItem shape are unchanged. Every error carries a versioned code string callers can pattern-match on.
• Upgradeable — tier-1 boundary intact: pptx.ts already routes through extractSlides(), so swapping in dom-to-pptx (tier 2) or a Chromium screenshot fallback only touches that one helper. chrome-discovery.ts accepts a deps injection point so a future "managed Chrome download" path can plug in without touching exporters.
• No bloat — three prod deps added (still well under the 30-prod-dep cap), all lazy-loaded so the cold-start bundle is unchanged. No Chromium ships with the installer.
• Elegant — PreviewToolbar keeps existing tokens / spacing / easing; only the per-item hint copy changed. Each exporter is a single small function with a single throw site.

How to test manually

1. pnpm i && pnpm dev
2. Generate any HTML preview, click Export, pick PDF → save dialog → confirm Chrome launches headless and a real PDF lands on disk.
3. Pick PPTX with a deck containing <section> blocks → opens in PowerPoint / Keynote with one slide per section.
4. Pick ZIP → archive contains index.html, README.md, and any assets.
5. With Chrome uninstalled (or set CODESIGN_CHROME_PATH=/nonexistent), PDF export shows the EXPORTER_NO_CHROME toast with the install link.