feat: implement milestone-based progress system and activate new lessons

Progress System:
- Replace percentage-based progress with milestone markers (1, 5, 10, 20, 30, 50, 75, 100)
- Add visual milestone indicators with reached/current/next states
- Add celebration animation when milestones are reached
- Update progress bar to show progress toward next milestone
- Add progressTextMilestone i18n key for all 6 languages

New Lessons Activated:
- HTML Dialog (native modal dialogs)
- HTML Progress & Meter (indicator elements)
- HTML Fieldset (form grouping)
- HTML Datalist (autocomplete inputs)

This adds 10 new lessons across all 6 languages, bringing total from ~66 to ~76.

src/index.html

@@ -468,11 +468,21 @@
 
 				<div class="sidebar-section">
 					<h4 data-i18n="progress">Progress</h4>
-					<div class="progress-display" id="progress-display">
+					<div class="progress-display milestone-progress" id="progress-display">
+						<div class="milestones" id="milestones">
+							<span class="milestone" data-value="1">1</span>
+							<span class="milestone" data-value="5">5</span>
+							<span class="milestone" data-value="10">10</span>
+							<span class="milestone" data-value="20">20</span>
+							<span class="milestone" data-value="30">30</span>
+							<span class="milestone" data-value="50">50</span>
+							<span class="milestone" data-value="75">75</span>
+							<span class="milestone" data-value="100">100</span>
+						</div>
 						<div class="progress-bar">
 							<div class="progress-fill" id="progress-fill"></div>
 						</div>
-						<span class="progress-text" id="progress-text">0% Complete</span>
+						<span class="progress-text" id="progress-text">0 of 100</span>
 					</div>
 				</div>