fix: board 404 returns clean page, not 500

- board.py: catch httpx.HTTPStatusError 404 and show 'Projet introuvable' page
- all functional tests pass: dashboard (50 unique), board, issues, checklists, notes, CSRF

app/routers/board.py

@@ -95,6 +95,14 @@ async def board(
             column_map[col].append(issue)
 
     except Exception as e:
+        from httpx import HTTPStatusError
+        if isinstance(e, HTTPStatusError) and e.response is not None and e.response.status_code == 404:
+            html = '<div style="padding:48px;text-align:center">'
+            html += '<h2 style="color:var(--text-dim)">Projet introuvable</h2>'
+            html += f'<p style="color:var(--text-dim)">{owner}/{repo} n\'existe pas sur Gitea.</p>'
+            html += '<a href="/" class="btn btn-primary" style="margin-top:16px">Retour aux projets</a>'
+            html += '</div>'
+            return HTMLResponse(html, status_code=404)
         logger.error("Board error: %s", e)
         return HTMLResponse(f"<p class='error'>Erreur: {e}</p>", status_code=500)