fix: Prevent frontend TypeError from undefined array access

Fix undefined variable crashes when accessing property 'length' on undefined:
- "can't access property 'length', e is undefined"

CHANGES:
1. Election detail page ([id]/page.tsx):
   - Ensure candidates array exists when fetching election data
   - Default to empty array if API returns undefined candidates

2. Active elections list page (active/page.tsx):
   - Validate API response is an array before processing
   - Ensure each election has candidates array with fallback

3. Blockchain visualizer (blockchain-visualizer.tsx):
   - Add optional chaining check before accessing data.blocks
   - Prevent crashes when data prop is undefined

All changes follow defensive programming practices:
- No more direct property access without null checks
- Array fallbacks ensure predictable behavior
- Optional chaining used consistently

This fixes the error that occurred when navigating to election pages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

e-voting-system/frontend/app/dashboard/votes/active/page.tsx

@@ -39,7 +39,12 @@ export default function ActiveVotesPage() {
         }
 
         const data = await response.json()
-        setElections(data || [])
+        // Ensure elections is an array and each has candidates
+        const elections = Array.isArray(data) ? data.map(e => ({
+          ...e,
+          candidates: e.candidates || []
+        })) : []
+        setElections(elections)
       } catch (err) {
         const message = err instanceof Error ? err.message : "Erreur lors du chargement"
         setError(message)