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/pricing.py

@@ -138,13 +138,16 @@ def compute_tariff(
         a1, a2, b2 = compute_integrals(vol, vinf, vmax, pmax, a, b, c, d, e)
         household_bills.append(abo + (a1 + a2) * p0 + b2)
 
+    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 TariffResult(
         p0=p0,
-        curve_volumes=vv.tolist(),
-        curve_prices_m3=prix_m3.tolist(),
-        curve_bills_rp=bills_rp.tolist(),
-        curve_bills_rs=bills_rs.tolist(),
-        household_bills=household_bills,
+        curve_volumes=sanitize(vv),
+        curve_prices_m3=sanitize(prix_m3),
+        curve_bills_rp=sanitize(bills_rp),
+        curve_bills_rs=sanitize(bills_rs),
+        household_bills=sanitize(household_bills),
     )
 
 
@@ -169,7 +172,7 @@ def compute_impacts(
     Returns (p0, list of ImpactRow).
     """
     if reference_volumes is None:
-        reference_volumes = [30, 60, 90, 150, 300]
+        reference_volumes = [30, 60, 90, 150, 300, 600]
 
     p0 = compute_p0(households, recettes, abop, abos, vinf, vmax, pmax, a, b, c, d, e)