Refactor ContributionCalendar to use UTC for date calculations

- Updated date handling in ContributionCalendar to work entirely in UTC, ensuring consistency with Gitea's date representation.
- Adjusted logic for calculating the start date of the 52-week period and generating calendar days accordingly.
- Improved comments for clarity regarding the use of UTC in date manipulations.
- Cleaned up redundant code and ensured proper date formatting in the rendering of calendar days.
- Minor adjustments in the development.astro file to comment out a collapsible intro section for better organization.

src/pages/[lang]/development.astro

@@ -128,10 +128,10 @@ const averagePerDay = contributionDays > 0 ? (totalContributions / contributionD
       </p>
     </div>
 
-    <!-- Collapsible Intro -->
+    <!-- Collapsible Intro 
     <div class="backdrop-blur-sm bg-white/70 dark:bg-slate-900/70 rounded-xl shadow p-6 mb-8">
       <CollapsibleIntro text={t.development.intro} client:visible />
-    </div>
+    </div>-->
 
     <!-- Stats Overview -->
     <div class="grid md:grid-cols-3 gap-6 mb-8">