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/widgets/download-button.html

@@ -3,8 +3,8 @@
 <button
     id="download-button"
     class="fixed-btn download-btn no-print has-tooltip"
-    aria-label="{{if eq .Lang "es"}}Descargar PDF{{else}}Download as PDF{{end}}"
-    data-tooltip="{{if eq .Lang "es"}}Descargar PDF{{else}}Download as PDF{{end}}"
+    aria-label="{{.UI.Widgets.Download.AriaLabel}}"
+    data-tooltip="{{.UI.Widgets.Download.Tooltip}}"
     onclick="openPdfModal()"
     _="on mouseenter call syncPdfHover(true)
        on mouseleave call syncPdfHover(false)">