Add interactive citizen page with sidebar, display settings, and adaptive CSS

Major rework of the citizen-facing page:
- Chart + sidebar layout (auth/vote/countdown in right sidebar)
- DisplaySettings component (font size, chart density, color palettes)
- Adaptive CSS with clamp() throughout, responsive breakpoints at 480/768/1024
- Baseline charts zoomed on first tier for small consumption detail
- Marginal price chart with dual Y-axes (foyers left, €/m³ right)
- Key metrics banner (5 columns: recettes, palier, prix palier, prix médian, mon prix)
- Client-side p0/impacts computation, draggable median price bar
- Household dots toggle, vote overlay curves
- Auth returns volume_m3, vote captures submitted_at
- Cleaned header nav (removed Accueil/Super Admin for public visitors)
- Terminology: foyer for bills, électeur for votes
- 600m³ added to impact reference volumes
- Realistic seed votes (50 votes, 3 profiles)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/app/pages/login/admin.vue

@@ -22,6 +22,11 @@
           <span v-else>Se connecter</span>
         </button>
       </form>
+
+      <!-- Dev credentials hint -->
+      <div v-if="isDev" style="margin-top: 1rem; padding: 0.75rem; background: #fef3c7; border: 1px solid #f59e0b; border-radius: 6px; font-size: 0.8rem;">
+        <strong>Dev:</strong> superadmin@sejeteralo.fr / superadmin
+      </div>
     </div>
 
     <p style="text-align: center; margin-top: 1rem;">
@@ -39,6 +44,7 @@ const email = ref('')
 const password = ref('')
 const error = ref('')
 const loading = ref(false)
+const isDev = import.meta.dev
 
 async function login() {
   error.value = ''