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/components/layout/Header.tsx

@@ -31,7 +31,7 @@ export async function Header() {
           {isAuthenticated ? (
             <UserMenu
               userName={session.user?.name}
-              userEmail={session.user?.email!}
+              userEmail={session.user?.email ?? ''}
             />
           ) : (
             <Link