refactor: Extract all hardcoded content to JSON files

- Move all bilingual text from templates to UI JSON (labels, buttons, modals)
- Move skills summary paragraph to CV JSON with HTML support
- Add new UI sections: navigation, viewControls, sections, footer, portfolio,
  pdfModal, shortcutsModal, infoModal, widgets
- Update Go structs to match expanded JSON structure
- Add template.HTML type for CV.SkillsSummary field
- Add JSON content validation test (70-json-content-validation.test.mjs)

Templates now contain only structural logic (CSS classes, HTML attributes)
while all user-visible text loads from JSON files for proper i18n support.

templates/partials/sections/header.html

@@ -6,7 +6,7 @@
         <div class="cv-header-content">
             <div class="cv-header-left">
                 <h1 class="cv-name">Moreno Rubio, Juan Andrés</h1>
-                <p class="years-experience">{{.YearsOfExperience}} {{if eq .Lang "es"}}años de experiencia{{else}}years of experience{{end}}</p>
+                <p class="years-experience">{{.YearsOfExperience}} {{.UI.Sections.YearsOfExperience}}</p>
 
                 <!-- Photo positioned for mobile (centered between name and intro) -->
                 <div class="cv-photo">