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>

backend/app/engine/current_model.py

@@ -56,11 +56,14 @@ def compute_linear_tariff(
     price_m3_rp = abop / vv + p0
     price_m3_rs = abos / vv + p0
 
+    def sanitize(arr):
+        return [0.0 if (x != x or x == float('inf') or x == float('-inf')) else float(x) for x in arr]
+
     return LinearTariffResult(
         p0=p0,
-        curve_volumes=vv.tolist(),
-        curve_bills_rp=bills_rp.tolist(),
-        curve_bills_rs=bills_rs.tolist(),
-        curve_price_m3_rp=price_m3_rp.tolist(),
-        curve_price_m3_rs=price_m3_rs.tolist(),
+        curve_volumes=sanitize(vv),
+        curve_bills_rp=sanitize(bills_rp),
+        curve_bills_rs=sanitize(bills_rs),
+        curve_price_m3_rp=sanitize(price_m3_rp),
+        curve_price_m3_rs=sanitize(price_m3_rs),
     )