🚸 feat: redesign AI toolbar with two-zone layout and HoverCard shortcuts popover

- Split composer into AI zone (primary accent) and data actions zone (neutral)
- Move Attach/Generate to labeled footer bar below textarea (left/right aligned)
- Add info icon with HoverCard (hover preview) + Popover (pinned click) showing identical keyboard shortcuts content using shadcn HoverCard to fix theme inconsistency vs Tooltip
- Expose imperative triggerRef on ImagePicker for keyboard shortcut access
- Wire TooltipProvider in root layout; install shadcn kbd and hover-card
- Unauthenticated state shows locked CTA with real sign-in button weight
- Add behavioral contract tests for footer bar, info trigger, and zone layout

src/components/ui/hover-card.tsx

@@ -0,0 +1,44 @@
+"use client"
+
+import * as React from "react"
+import { HoverCard as HoverCardPrimitive } from "radix-ui"
+
+import { cn } from "@/lib/utils"
+
+function HoverCard({
+  ...props
+}: React.ComponentProps<typeof HoverCardPrimitive.Root>) {
+  return <HoverCardPrimitive.Root data-slot="hover-card" {...props} />
+}
+
+function HoverCardTrigger({
+  ...props
+}: React.ComponentProps<typeof HoverCardPrimitive.Trigger>) {
+  return (
+    <HoverCardPrimitive.Trigger data-slot="hover-card-trigger" {...props} />
+  )
+}
+
+function HoverCardContent({
+  className,
+  align = "center",
+  sideOffset = 4,
+  ...props
+}: React.ComponentProps<typeof HoverCardPrimitive.Content>) {
+  return (
+    <HoverCardPrimitive.Portal data-slot="hover-card-portal">
+      <HoverCardPrimitive.Content
+        data-slot="hover-card-content"
+        align={align}
+        sideOffset={sideOffset}
+        className={cn(
+          "z-50 w-64 origin-(--radix-hover-card-content-transform-origin) rounded-md border bg-popover p-4 text-popover-foreground shadow-md outline-hidden data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2 data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=closed]:zoom-out-95 data-[state=open]:animate-in data-[state=open]:fade-in-0 data-[state=open]:zoom-in-95",
+          className
+        )}
+        {...props}
+      />
+    </HoverCardPrimitive.Portal>
+  )
+}
+
+export { HoverCard, HoverCardTrigger, HoverCardContent }