feat(design): the button families on the decided ladder, first half of the sweep (E28 S3b1)

Button is rebuilt to A1/B1/B3: explicit heights (xs 24 / sm 28 / md 32, lg 40 for a hero CTA), r8 (a
pill on media), state layers instead of opacity and bg-card hovers, a `pressed` toggle, `overlay`,
`media` and `link-muted` variants. The icon-only sizes move to the new IconButton (28 / 32, 48 only for
the Play on a picture; a required label). Pressable (a card, row or tile that is the control) and
DragHandle join them. The state-* utilities now stand down on a disabled, busy or pressed control, held
by theme.contract.test.ts.

Swept onto them: the Button icon-size callers and 16 files (Plex playlist, Plex detail, the YT player
modal chrome, Welcome, Plex and video cards with AddToPlaylist and DownloadButton, the video editor,
Feed, personal views, Scheduler, Users, the nav rail, Plex detail parts, Settings): 268 raw buttons are
153. Fixed on the way: FocusSafeButton overwrote a caller's aria-disabled (the "unavailable, here is
why" state; adminInvites caught it), a season toggle and a Plex settings switch had no name/state, and
the video card's overlay follows the C5 card (Resume pill, From the start pill, a round Play).

frontend/src/components/Messages.tsx

@@ -6,6 +6,7 @@ import { ArrowLeft, ShieldCheck, SquarePen, ExternalLink } from "lucide-react";
 import { api, type Conversation, type MessageUser } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { Button } from "./ui/Button";
+import { IconButton } from "./ui/IconButton";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import { relativeTime } from "../lib/format";
 import { partnerPub, POLL_MS, SYSTEM_ID, useKeyState } from "../lib/messaging";
@@ -221,15 +222,16 @@ function Directory({ onPick, onBack }: { onPick: (u: MessageUser) => void; onBac
   return (
     <div className="space-y-3">
       <div className="flex items-center gap-2">
-        <Button
+        <IconButton
+          label={t("channel.back")}
+          size="sm"
           variant="ghost"
-          size="icon-sm"
           // An icon-only button needs a name: both of these announced themselves as a bare "button",
           // and they are the only way out of the sub-view (E10 review round 1).
-          label={t("channel.back")}
           onClick={onBack}
-          icon={<ArrowLeft className="w-4 h-4" />}
-        />
+        >
+          <ArrowLeft className="w-4 h-4" />
+        </IconButton>
         <div className="text-xs uppercase tracking-wide text-muted">{t("messages.newTitle")}</div>
       </div>
       <input
@@ -291,15 +293,16 @@ function PageThread({
   return (
     <div className="flex flex-col h-[65vh] min-h-80">
       <div className="flex items-center gap-3 pb-3 border-b border-border">
-        <Button
+        <IconButton
+          label={t("channel.back")}
+          size="sm"
           variant="ghost"
-          size="icon-sm"
           // An icon-only button needs a name: both of these announced themselves as a bare "button",
           // and they are the only way out of the sub-view (E10 review round 1).
-          label={t("channel.back")}
           onClick={onBack}
-          icon={<ArrowLeft className="w-4 h-4" />}
-        />
+        >
+          <ArrowLeft className="w-4 h-4" />
+        </IconButton>
         <Avatar
           src={seedAvatar}
           fallback={name}