refactor: Rename 'extended' → 'long' + add compact sidebar fonts

BREAKING CHANGE: API parameter renamed from 'extended' to 'long'

## Breaking Change: Terminology Standardization

Renamed 'extended' to 'long' across entire codebase for consistency:

**Backend (Go):**
- internal/handlers/cv.go (7 locations)
  - Migration logic to auto-convert 'extended' → 'long' cookies
  - API validation now rejects 'extended', requires 'long'
  - Toggle state logic updated
- internal/handlers/pdf_test.go (17 occurrences)
  - Test function renamed: TestExportPDF_ExtendedWithSkills → TestExportPDF_LongWithSkills
  - All test cases, parameters, and expected filenames updated
- internal/pdf/generator.go (2 comment updates)

**Frontend:**
- PDF-EXPORT-FEATURE.md (3 occurrences)
- doc/3-API.md (parameter documentation)
- doc/7-CUSTOMIZATION.md (examples updated)
- templates/partials/modals/pdf-modal.html (button text, URLs)
- static/js/main.js (migration logic)
- static/hyperscript/toggles._hs (toggle logic)
- tests/mjs/24-pdf-download-params.test.mjs (test expectations)
- tests/mjs/test-preference-migration.test.mjs (NEW)
- tests/mjs/verify-migration.test.mjs (NEW)

**PDFs Renamed:**
- cv-extended-with_skills-jamr-2025-en.pdf → cv-long-with_skills-jamr-2025-en.pdf
- cv-extended-with_skills-jamr-2025-es.pdf → cv-long-with_skills-jamr-2025-es.pdf

**Migration:** Automatic cookie migration from 'extended' → 'long' for seamless UX

## New Feature: Compact Sidebar Fonts

Reduces page count for short CV with skills from 6 → 5 pages:

**Implementation:**
- Location: internal/pdf/generator.go (lines 154-215)
- Cookie detection: `cookies["cv-length"] == "short"`
- Font reduction: 2-6% (0.94-0.98em) - very subtle
- Only activates for: `length=short` + `version=with_skills`
- Long version: Always uses full-size fonts

**Impact:**
- Page count: 6 pages → 5 pages (16.7% reduction)
- Readability: Maintained - fonts remain professional
- Design philosophy: Subtle, natural content flow

**Testing:**
- New test: TestPDFGenerator_CompactSidebarFonts
- Comprehensive coverage of cookie detection and PDF generation
- Manual verification: 5-page PDF with compact but readable fonts

**Documentation:**
- doc/LONG-PDF-GENERATION.md (NEW, 13 KB)
  - Complete feature documentation
  - Implementation details with code examples
  - Font size breakdown table
  - Testing and troubleshooting guides
  - Compact sidebar fonts section (comprehensive)

**Files Changed:**
- 11 modified (backend + frontend + docs)
- 5 new files (2 PDFs, 1 doc, 2 tests)
- 2 files renamed (PDFs)

**Tests:** All Go tests passing, API validation verified, PDF generation tested

doc/7-CUSTOMIZATION.md

@@ -1195,7 +1195,7 @@ The PDF export endpoint accepts 4 parameters that let users customize their PDF:
 http://localhost:1999/export/pdf?lang=en&length=short&version=clean
 
 # Long, detailed Spanish CV with all icons
-http://localhost:1999/export/pdf?lang=es&length=long&icons=show&version=extended
+http://localhost:1999/export/pdf?lang=es&length=long&icons=show&version=long
 
 # Use defaults (English, short, with icons, extended)
 http://localhost:1999/export/pdf
@@ -1274,7 +1274,7 @@ Add a download button to your templates:
 
 1. **Keep it concise**: Use `length=short` for 1-2 page CVs
 2. **Professional look**: Use `version=clean&icons=hide` for formal applications
-3. **Colorful**: Use `version=extended&icons=show` for creative industries
+3. **Colorful**: Use `version=long&icons=show` for creative industries
 4. **Test before sharing**: Always preview the PDF before sending to employers
 5. **File size**: Short versions ~1.5-2MB, Long versions ~2-2.5MB