Add Gantt chart view with card dependencies and start dates

Adds a new Gantt chart board view with custom SVG timeline rendering,
card-to-card dependency tracking with cycle detection, and a startDate
field on cards. Includes zoom controls (day/week/month), dependency
arrows, column-grouped task list, today marker, and real-time updates.

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

prisma/schema.prisma

@@ -165,6 +165,7 @@ model Card {
   description     String?
   position        String    // Fractional index (lexicographic)
   dueDate         DateTime? @map("due_date")
+  startDate       DateTime? @map("start_date")
   archived        Boolean   @default(false)
   dueReminderSent Boolean   @default(false) @map("due_reminder_sent")
   cardNumber      Int?      @map("card_number")
@@ -180,6 +181,8 @@ model Card {
   activities       Activity[]
   watchers         CardWatcher[]
   customFieldValues CustomFieldValue[]
+  blocks           CardDependency[] @relation("CardBlocks")
+  blockedBy        CardDependency[] @relation("CardBlockedBy")
 
   @@map("cards")
 }
@@ -458,6 +461,21 @@ model AutomationRule {
   @@map("automation_rules")
 }
 
+// ─── Card Dependencies ──────────────────────────────────────
+
+model CardDependency {
+  id             String   @id @default(uuid()) @db.Uuid
+  blockingCardId String   @map("blocking_card_id") @db.Uuid
+  blockedCardId  String   @map("blocked_card_id") @db.Uuid
+  createdAt      DateTime @default(now()) @map("created_at")
+
+  blockingCard Card @relation("CardBlocks", fields: [blockingCardId], references: [id], onDelete: Cascade)
+  blockedCard  Card @relation("CardBlockedBy", fields: [blockedCardId], references: [id], onDelete: Cascade)
+
+  @@unique([blockingCardId, blockedCardId])
+  @@map("card_dependencies")
+}
+
 // ─── Tenant Branding & Theming ───────────────────────────────
 
 model TenantTheme {