feat: Add loading overlay feedback for PDF downloads

UX Improvement:
- Added visual feedback during PDF generation process
- Users now see immediate response when clicking download button
- Clear communication about what's happening during the wait

New Features:
- Loading overlay with animated spinner
- Format-specific estimated generation times (Short: ~3s, Default: ~4s, Long: ~8s)
- Blur effect on modal background during loading
- Bilingual support (English/Spanish)
- Automatic modal close after download completes

CSS Updates (static/css/04-interactive/_modals.css):
- Added .pdf-loading-overlay with glassmorphism effect
- Spinning animation for loader (1s linear infinite)
- Fade-in animation (300ms)
- Accessibility: respects prefers-reduced-motion
- Background blur when loading active

HTML Updates (templates/partials/modals/pdf-modal.html):
- Loading overlay structure with spinner
- Dynamic loading messages based on selected format
- Enhanced downloadPDF() function with timing logic

Before: Click → silence → download appears
After: Click → overlay + spinner + estimate → download appears

static/css/04-interactive/_modals.css

@@ -769,4 +769,96 @@
     .pdf-download-btn {
         transition: none;
     }
+
+    .pdf-loading-overlay {
+        animation: none;
+    }
+
+    .pdf-loading-spinner {
+        animation: none;
+    }
+}
+
+/* =============================================================================
+   PDF LOADING OVERLAY - Visual Feedback During Download
+   ============================================================================= */
+
+.pdf-loading-overlay {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    background: rgba(255, 255, 255, 0.95);
+    backdrop-filter: blur(8px);
+    -webkit-backdrop-filter: blur(8px);
+    border-radius: 24px;
+    display: none;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    z-index: 100;
+    animation: overlayFadeIn 0.3s ease;
+}
+
+.pdf-loading-overlay.active {
+    display: flex;
+}
+
+@keyframes overlayFadeIn {
+    from {
+        opacity: 0;
+    }
+    to {
+        opacity: 1;
+    }
+}
+
+.pdf-loading-content {
+    text-align: center;
+    max-width: 300px;
+    padding: 2rem;
+}
+
+.pdf-loading-spinner {
+    width: 64px;
+    height: 64px;
+    margin: 0 auto 1.5rem;
+    border: 4px solid rgba(239, 68, 68, 0.2);
+    border-top-color: #ef4444;
+    border-radius: 50%;
+    animation: spin 1s linear infinite;
+}
+
+@keyframes spin {
+    to {
+        transform: rotate(360deg);
+    }
+}
+
+.pdf-loading-title {
+    font-size: 1.25rem;
+    font-weight: 600;
+    color: var(--text-primary);
+    margin: 0 0 0.5rem 0;
+}
+
+.pdf-loading-message {
+    font-size: 0.95rem;
+    color: var(--text-gray);
+    margin: 0 0 0.5rem 0;
+    line-height: 1.5;
+}
+
+.pdf-loading-estimate {
+    font-size: 0.85rem;
+    color: #999;
+    font-style: italic;
+    margin: 0;
+}
+
+/* Blur the modal content when overlay is active */
+.info-modal-content.loading-active > *:not(.pdf-loading-overlay) {
+    filter: blur(3px);
+    pointer-events: none;
 }