feat(pdf): complete Epic 3 - PDF Export functionality

Story 3-1: Create PDF Generation API Route
- Add server/api/resume/pdf.get.ts with Puppeteer integration
- Support both dev (puppeteer) and prod (puppeteer-core + chromium)
- Navigate to /resume?print=true for WYSIWYG capture
- Return PDF with proper headers (Content-Type, Content-Disposition)
- Add error handling with 500 status and JSON response
- Update vercel.json with function config (memory: 1024, maxDuration: 10)

Story 3-2: Create PDF Download Composable
- Add app/composables/useResumePdf.ts
- Implement isGenerating ref for loading state
- Implement downloadPdf() with blob handling
- Add toast notifications for errors
- Revoke object URL to prevent memory leaks

Story 3-3: Connect Download Button to PDF Generation
- Update ResumeDownloadButton.vue to use useResumePdf()
- Bind :loading and :disabled to isGenerating
- Connect @click to downloadPdf
- Remove placeholder handler

Dependencies Added:
- puppeteer ^24.31.0
- puppeteer-core ^24.31.0
- @sparticuz/chromium ^141.0.0

Closes Epic 3
Closes Story 3-1, 3-2, 3-3

docs/sprint-artifacts/sprint-status.yaml

@@ -61,7 +61,7 @@ development_status:
   # FRs: FR10-14
   # ═══════════════════════════════════════════════════════════════
   epic-3: contexted
-  3-1-create-pdf-generation-api-route: ready-for-dev
-  3-2-create-pdf-download-composable: ready-for-dev
-  3-3-connect-download-button-to-pdf-generation: ready-for-dev
+  3-1-create-pdf-generation-api-route: done
+  3-2-create-pdf-download-composable: done
+  3-3-connect-download-button-to-pdf-generation: done
   epic-3-retrospective: optional