feat(list): click sender avatar to select message/thread (Thunderbird-style)

Wrap the message-list avatar in a SelectableAvatar control: clicking the
avatar toggles the row into the current selection instead of opening it,
matching Thunderbird's correspondent-avatar selection affordance. A check
overlay appears on hover (hinting it is clickable) and stays while selected.

- email-list-item + thread single-email: toggle that message's id
- thread header: toggle the whole thread (reuses existing thread-select logic)
- focused-mail and extra-compact layouts render no avatar, so unaffected

components/email/selectable-avatar.tsx

@@ -0,0 +1,59 @@
+"use client";
+
+import type { ComponentProps } from "react";
+import { Check } from "lucide-react";
+import { Avatar } from "@/components/ui/avatar";
+import { cn } from "@/lib/utils";
+
+type SelectableAvatarProps = ComponentProps<typeof Avatar> & {
+  /** Whether the underlying message/thread is currently selected. */
+  checked: boolean;
+  /** Toggle selection. The wrapper stops propagation so the row is not opened. */
+  onToggle: () => void;
+  /** Accessible label for the selection control. */
+  selectLabel?: string;
+};
+
+/**
+ * Avatar that doubles as a selection control, Thunderbird-style: clicking the
+ * avatar toggles the message/thread into the current selection instead of
+ * opening it. A check overlay appears on hover (hinting it is clickable) and
+ * stays visible while the row is selected.
+ */
+export function SelectableAvatar({
+  checked,
+  onToggle,
+  selectLabel,
+  className,
+  ...avatarProps
+}: SelectableAvatarProps) {
+  return (
+    <button
+      type="button"
+      role="checkbox"
+      aria-checked={checked}
+      aria-label={selectLabel}
+      onClick={(e) => {
+        e.stopPropagation();
+        onToggle();
+      }}
+      className={cn(
+        "group/select relative shrink-0 rounded-full outline-none",
+        "focus-visible:ring-2 focus-visible:ring-primary/60",
+        className,
+      )}
+    >
+      <Avatar {...avatarProps} />
+      <span
+        aria-hidden
+        className={cn(
+          "absolute inset-0 flex items-center justify-center rounded-full",
+          "bg-primary text-primary-foreground transition-opacity duration-150",
+          checked ? "opacity-100" : "opacity-0 group-hover/select:opacity-100",
+        )}
+      >
+        <Check className="h-4 w-4" />
+      </span>
+    </button>
+  );
+}