feat(plan): add Plan tab as the first project surface

A new home for everything that happens BEFORE building:
- Vision    — one-line elevator pitch (mirrors productVision)
- Ideas     — the "park-it" bin for raw thoughts
- Tasks     — what needs to happen next (open / done)
- Decisions — log of "we chose X over Y because Z"

Storage is appended under fs_projects.data.plan so no schema migration
is needed. CRUD lives at /api/projects/[projectId]/plan.

The bare project URL now redirects to /plan instead of /product, and
the AI chat receives decisions + open tasks in its active-project
context block — so it stops re-litigating settled questions and knows
what's queued up.

Made-with: Cursor

components/project/project-tab-bar.tsx

@@ -10,12 +10,13 @@
 
 import Link from "next/link";
 import { usePathname } from "next/navigation";
-import { Box, Cloud, Server } from "lucide-react";
+import { Box, Cloud, Server, NotebookPen } from "lucide-react";
 
 const TABS = [
-  { id: "product",        label: "Product",        icon: Box,    blurb: "Custom code, design, and content built for this vision." },
-  { id: "infrastructure", label: "Infrastructure", icon: Server, blurb: "Swappable services this product depends on." },
-  { id: "hosting",        label: "Hosting",        icon: Cloud,  blurb: "Where it runs and how people reach it." },
+  { id: "plan",           label: "Plan",           icon: NotebookPen, blurb: "Vision, ideas, tasks, and decisions for this project." },
+  { id: "product",        label: "Product",        icon: Box,         blurb: "Custom code, design, and content built for this vision." },
+  { id: "infrastructure", label: "Infrastructure", icon: Server,      blurb: "Swappable services this product depends on." },
+  { id: "hosting",        label: "Hosting",        icon: Cloud,       blurb: "Where it runs and how people reach it." },
 ] as const;
 
 export function ProjectTabBar({
@@ -28,7 +29,7 @@ export function ProjectTabBar({
   const pathname = usePathname() ?? "";
   const activeTab =
     TABS.find(t => pathname.includes(`/project/${projectId}/${t.id}`))?.id ??
-    "product";
+    "plan";
 
   return (
     <nav style={tabBar} aria-label="Project sections">