feat(coach): tap-friendly composer hint and accurate evidence aria-expanded

- New `<Popover>` primitive (Radix-backed, Dracula surface tokens)
  mirrors the existing `<Tooltip>` wrapper. Tooltips do not
  tap-toggle reliably on touch; popovers do.
- The Coach composer info-icon swaps off `<Tooltip>` onto
  `<Popover>` so the long-form "Enter to send, Shift+Enter for a new
  line" copy is reachable via a tap on mobile while the keyboard
  hover-preview semantic is preserved on desktop. The `aria-label`
  on the trigger keeps screen-reader users announced.
- The composer textarea now carries `enterKeyHint="send"`,
  `autoCapitalize="sentences"`, and an opt-in `autoFocusOnOpen` prop.
  The drawer flips that prop on when it mounts the fallback
  composer, so the textarea claims the cursor the first time the
  drawer opens; subsequent re-renders of an already-open drawer do
  not steal focus back.
- Coach evidence `<details>` is now controlled by local state. The
  summary carries an accurate `aria-expanded` that flips in
  lock-step with the disclosure; the native Enter / Space toggle
  semantics are preserved.

src/components/ui/popover.tsx

@@ -0,0 +1,55 @@
+"use client";
+
+import * as React from "react";
+import { Popover as PopoverPrimitive } from "radix-ui";
+
+import { cn } from "@/lib/utils";
+
+/**
+ * Popover primitive — Radix `Popover` wrapped with the same Dracula
+ * surface tokens the rest of the shadcn primitives use.
+ *
+ * v1.4.27 MB3 — introduced so the Coach composer hint can swap off
+ * the `<Tooltip>` primitive. Radix tooltips never tap-toggle reliably
+ * on touch surfaces; popovers do. The mouse-hover preview that the
+ * tooltip used to provide on desktop is dropped intentionally —
+ * tap-to-open is the same gesture on every device.
+ */
+function Popover({
+  ...props
+}: React.ComponentProps<typeof PopoverPrimitive.Root>) {
+  return <PopoverPrimitive.Root data-slot="popover" {...props} />;
+}
+
+function PopoverTrigger({
+  ...props
+}: React.ComponentProps<typeof PopoverPrimitive.Trigger>) {
+  return <PopoverPrimitive.Trigger data-slot="popover-trigger" {...props} />;
+}
+
+function PopoverContent({
+  className,
+  align = "center",
+  sideOffset = 4,
+  children,
+  ...props
+}: React.ComponentProps<typeof PopoverPrimitive.Content>) {
+  return (
+    <PopoverPrimitive.Portal>
+      <PopoverPrimitive.Content
+        data-slot="popover-content"
+        align={align}
+        sideOffset={sideOffset}
+        className={cn(
+          "bg-popover text-popover-foreground border-border animate-in fade-in-0 zoom-in-95 data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=closed]:zoom-out-95 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 z-50 max-w-xs origin-(--radix-popover-content-transform-origin) rounded-md border p-3 text-xs shadow-md outline-none",
+          className,
+        )}
+        {...props}
+      >
+        {children}
+      </PopoverPrimitive.Content>
+    </PopoverPrimitive.Portal>
+  );
+}
+
+export { Popover, PopoverTrigger, PopoverContent };