feat: add weather trend chart showing indicator averages over time

Adds a collapsible SVG line graph on weather session pages displaying
the evolution of all 4 indicators (Performance, Moral, Flux, Création
de valeur) across sessions, with per-session average scores, hover
tooltips, and a marker on the current session.

Also fixes pre-existing lint errors: non-null assertion on optional
chain in Header and eslint-disable for intentional hydration pattern
in ThemeToggle.

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

src/services/weather.ts

@@ -8,8 +8,19 @@ import {
   getSessionByIdGeneric,
 } from '@/services/session-queries';
 import { getWeekBounds } from '@/lib/date-utils';
+import { getEmojiScore } from '@/lib/weather-utils';
 import type { ShareRole } from '@prisma/client';
 
+export type WeatherHistoryPoint = {
+  sessionId: string;
+  title: string;
+  date: Date;
+  performance: number | null;
+  moral: number | null;
+  flux: number | null;
+  valueCreation: number | null;
+};
+
 const weatherInclude = {
   user: { select: { id: true, name: true, email: true } },
   shares: { include: { user: { select: { id: true, name: true, email: true } } } },
@@ -343,3 +354,58 @@ export type WeatherSessionEventType =
 export const createWeatherSessionEvent = weatherShareEvents.createEvent;
 export const getWeatherSessionEvents = weatherShareEvents.getEvents;
 export const getLatestWeatherEventTimestamp = weatherShareEvents.getLatestEventTimestamp;
+
+// ============================================
+// Weather History (for trend chart)
+// ============================================
+
+function avgScore(emojis: (string | null)[]): number | null {
+  const scores = emojis.map(getEmojiScore).filter((s): s is number => s !== null);
+  if (scores.length === 0) return null;
+  return scores.reduce((sum, s) => sum + s, 0) / scores.length;
+}
+
+export async function getWeatherSessionsHistory(userId: string): Promise<WeatherHistoryPoint[]> {
+  const entrySelect = {
+    performanceEmoji: true,
+    moralEmoji: true,
+    fluxEmoji: true,
+    valueCreationEmoji: true,
+  } as const;
+
+  const [ownSessions, sharedRaw] = await Promise.all([
+    prisma.weatherSession.findMany({
+      where: { userId },
+      select: { id: true, title: true, date: true, entries: { select: entrySelect } },
+    }),
+    prisma.weatherSessionShare.findMany({
+      where: { userId },
+      select: {
+        session: {
+          select: { id: true, title: true, date: true, entries: { select: entrySelect } },
+        },
+      },
+    }),
+  ]);
+
+  const seen = new Set<string>();
+  const all: { id: string; title: string; date: Date; entries: typeof ownSessions[0]['entries'] }[] = [];
+  for (const s of [...ownSessions, ...sharedRaw.map((r) => r.session)]) {
+    if (!seen.has(s.id)) {
+      seen.add(s.id);
+      all.push(s);
+    }
+  }
+
+  all.sort((a, b) => a.date.getTime() - b.date.getTime());
+
+  return all.map((s) => ({
+    sessionId: s.id,
+    title: s.title,
+    date: s.date,
+    performance: avgScore(s.entries.map((e) => e.performanceEmoji)),
+    moral: avgScore(s.entries.map((e) => e.moralEmoji)),
+    flux: avgScore(s.entries.map((e) => e.fluxEmoji)),
+    valueCreation: avgScore(s.entries.map((e) => e.valueCreationEmoji)),
+  }));
+}