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/utils/bezier-math.ts

@@ -277,7 +277,7 @@ export function computeImpacts(
   c: number,
   d: number,
   e: number,
-  referenceVolumes: number[] = [30, 60, 90, 150, 300],
+  referenceVolumes: number[] = [30, 60, 90, 150, 300, 600],
 ): { p0: number; impacts: ImpactRow[] } {
   const p0 = computeP0(households, recettes, abop, abos, vinf, vmax, pmax, a, b, c, d, e)