feat: implement Story 1.1 — create and view diagrams

Add diagram/project DB schema, CRUD API, dashboard pages with grid/card/
empty state, create dialog with type selector, editor placeholder, and
26 schema validation tests. Includes code review fixes: soft-delete
filter on GET /:id, error handling, keyboard accessibility, type-safe
API response types, and error states on pages.

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

packages/ui/web/src/components/icons.tsx

@@ -149,6 +149,10 @@ import {
   ScrollText,
   Mic,
   MicOff,
+  Workflow,
+  Server,
+  ArrowRightLeft,
+  GitBranch,
 } from "lucide-react";
 
 import { Icons as GlobalIcons } from "@turbostarter/ui/assets";
@@ -373,6 +377,10 @@ export const Icons = {
   ScrollText,
   Mic,
   MicOff,
+  Workflow,
+  Server,
+  ArrowRightLeft,
+  GitBranch,
   MinusIcon: Minus,
   PlusIcon: Plus,
   // AI provider icons