feat: teeth tracker, feeding analysis, sidebar groups, timeline polish

Features:
- Teeth tracker: Tooth model + migration, API routes (GET/POST upsert/DELETE),
  page at /teeth with 20-tooth grid, progress bar, inline mark/unmark flow
- Dashboard feeding analysis: avg interval + last feed + next expected time widget
- Fix dashboard baby?.name crash (selectedBaby?.name)

UI/UX:
- Sidebar: grouped into Suivi / Santé / Vie du bébé / Outils sections
- Nav: active highlight works on child routes (startsWith)
- Timeline: filter chips sticky on mobile; skeleton cards replace spinner
- Stats: 500-event limit warning banner
- Medications: 3 states — "Dans Xh" (orange), "Disponible" (green), "En retard de Xh" (amber)
- Notes: skip auto-save on empty/whitespace blur

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

prisma/schema.prisma

@@ -136,6 +136,7 @@ model Baby {
   vaccinations          Vaccination[]
   medicationReminders   MedicationReminder[]
   milkStocks            MilkStock[]
+  teeth                 Tooth[]
   createdAt             DateTime     @default(now())
 }
 
@@ -279,3 +280,16 @@ model MedicationReminder {
   lastSentAt    DateTime?
   createdAt     DateTime @default(now())
 }
+
+model Tooth {
+  id         String   @id @default(cuid())
+  babyId     String
+  baby       Baby     @relation(fields: [babyId], references: [id], onDelete: Cascade)
+  code       String
+  appearedAt DateTime
+  notes      String?
+  createdAt  DateTime @default(now())
+  updatedAt  DateTime @updatedAt
+
+  @@unique([babyId, code])
+}