feat(web): add workspace management UI (M2 #12)

- Create workspace listing page at /settings/workspaces
  - List all user workspaces with role badges
  - Create new workspace functionality
  - Display member count per workspace

- Create workspace detail page at /settings/workspaces/[id]
  - Workspace settings (name, ID, created date)
  - Member management with role editing
  - Invite member functionality
  - Delete workspace (owner only)

- Add workspace components:
  - WorkspaceCard: Display workspace info with role badge
  - WorkspaceSettings: Edit workspace settings and delete
  - MemberList: Display and manage workspace members
  - InviteMember: Send invitations with role selection

- Add WorkspaceMemberWithUser type to shared package
- Follow existing app patterns for styling and structure
- Use mock data (ready for API integration)

apps/api/prisma/schema.prisma

@@ -144,10 +144,24 @@ model User {
   relationships        Relationship[]    @relation("RelationshipCreator")
   agentSessions        AgentSession[]
   userLayouts          UserLayout[]
+  userPreference       UserPreference?
 
   @@map("users")
 }
 
+model UserPreference {
+  id        String   @id @default(uuid()) @db.Uuid
+  userId    String   @unique @map("user_id") @db.Uuid
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  theme     String   @default("system")
+  locale    String   @default("en")
+  timezone  String?
+  settings  Json     @default("{}")
+  updatedAt DateTime @updatedAt @map("updated_at") @db.Timestamptz
+
+  @@map("user_preferences")
+}
+
 model Workspace {
   id        String   @id @default(uuid()) @db.Uuid
   name      String