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/education.html

@@ -7,12 +7,12 @@
             <summary>
                 <h3 class="section-title">
                     <iconify-icon icon="mdi:school" width="24" height="24" class="section-icon"></iconify-icon>
-                    {{if eq .Lang "es"}}Formación{{else}}Training{{end}}
+                    {{.UI.Navigation.Training}}
                 </h3>
             </summary>
         {{range .CV.Education}}
         <div class="education-item">
-            <strong>{{.Degree}}</strong> ({{.StartDate}}-{{.EndDate}}) {{if eq $.Lang "es"}}obtenido de{{else}}obtained from the{{end}} <strong>{{.Institution}}</strong> ({{.Location}})
+            <strong>{{.Degree}}</strong> ({{.StartDate}}-{{.EndDate}}) {{$.UI.Sections.ObtainedFrom}} <strong>{{.Institution}}</strong> ({{.Location}})
         </div>
         {{end}}
         </details>