feat: enhance experience section with icons, duration, and improved styling

Experience Section Improvements:
- Increased company logo size from 48px to 64px
- Added default office building icon for companies without logos
- Increased spacing between entries (2.5rem margin, 2rem padding)
- Added visible separator lines (2px solid #ddd)
- Made dates bold (weight: 600) and larger (1.05rem)
- Changed date color to #555 for better contrast

Dynamic Duration Calculation:
- Added automatic years/months calculation for each position
- Format: "(4 years 10 months)", "(2 years)", "(6 months)"
- Smart pluralization in English and Spanish
- Handles current positions (calculates to today)
- Added Duration field to Experience model

Iconify Integration:
- Added Iconify library (v3.1.1) for icon management
- Replaced EN/ES text with round flag icons (circle-flags:us, circle-flags:es)
- Updated CV site icon to mdi:file-account
- Replaced toggle text with intuitive icons:
  * Short/Long: mdi:file-document-outline / mdi:file-document-multiple-outline
  * Logos: mdi:image-off-outline / mdi:image-multiple-outline
- Default company icon: mdi:office-building (64x64px, light gray background)

Logo Display:
- Logos now show by default (toggle checked on page load)
- Toggle controls icon visibility
- Consistent spacing with default icon placeholder

Files modified:
- internal/handlers/cv.go: Added calculateDuration() function
- internal/models/cv.go: Added Duration field to Experience struct
- templates/index.html: Iconify integration, flag icons, toggle icons
- templates/cv-content.html: Duration display, default icon logic
- static/css/main.css: Bold dates, larger font sizes
- static/css/logo-toggle.css: Icon styling, separator lines, spacing

static/css/main.css

@@ -437,11 +437,16 @@ a:hover {
 
 .experience-period,
 .experience-separator,
-.experience-location {
-    color: #aaa;
-    font-weight: 500;
+.experience-location,
+.experience-duration {
+    color: #555;
+    font-weight: 600;
     display: inline-block;
-    font-size: 0.9rem;
+    font-size: 1.05rem;
+}
+
+.experience-duration {
+    font-style: italic;
 }
 
 .short-desc {