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/components/charts/VoteOverlayChart.vue

@@ -1,6 +1,12 @@
 <template>
   <div class="overlay-chart">
     <svg :viewBox="`0 0 ${svgW} ${svgH}`" preserveAspectRatio="xMidYMid meet">
+      <defs>
+        <clipPath id="overlay-clip">
+          <rect :x="margin.left" :y="margin.top" :width="plotW" :height="plotH" />
+        </clipPath>
+      </defs>
+
       <!-- Grid -->
       <g>
         <line
@@ -17,16 +23,19 @@
         />
       </g>
 
-      <!-- Vote curves (semi-transparent) -->
-      <g v-for="(vote, i) in votes" :key="i">
-        <path :d="getVotePath(vote, 1)" fill="none" stroke="#3b82f6" stroke-width="1.5" opacity="0.4" />
-        <path :d="getVotePath(vote, 2)" fill="none" stroke="#ef4444" stroke-width="1.5" opacity="0.4" />
-      </g>
+      <!-- Clipped curves -->
+      <g clip-path="url(#overlay-clip)">
+        <!-- Vote curves (semi-transparent) -->
+        <g v-for="(vote, i) in votes" :key="i">
+          <path :d="getVotePath(vote, 1)" fill="none" stroke="#3b82f6" stroke-width="1.5" opacity="0.4" />
+          <path :d="getVotePath(vote, 2)" fill="none" stroke="#ef4444" stroke-width="1.5" opacity="0.4" />
+        </g>
 
-      <!-- Median curve (if available) -->
-      <g v-if="medianVote">
-        <path :d="getVotePath(medianVote, 1)" fill="none" stroke="#1e40af" stroke-width="4" />
-        <path :d="getVotePath(medianVote, 2)" fill="none" stroke="#991b1b" stroke-width="4" />
+        <!-- Median curve (if available) -->
+        <g v-if="medianVote">
+          <path :d="getVotePath(medianVote, 1)" fill="none" stroke="#1e40af" stroke-width="4" />
+          <path :d="getVotePath(medianVote, 2)" fill="none" stroke="#991b1b" stroke-width="4" />
+        </g>
       </g>
 
       <!-- Axis labels -->