feat(e10): migrate the notification, message, auth and Plex action buttons

Second sweep, 34 more sites: NotificationsPanel (14 -- eight identical
accent text links, three ghost icon buttons, two ghost text actions),
Welcome's five auth links, KeyGate, Messages, NavSidebar, ChatThread,
ShareDialog, ChannelPage, PlexPlaylistView, PlexPlaylistAdd,
PlexCollectionEditor and plexViews.

Moves the font-weight out of Button's BASE and into each variant, so the
`link` variant can be semibold (which is what every text link in this app
looks like) while the action variants stay medium, and no element can end
up with two font-* utilities.

Welcome is the one signed-out surface this session cannot see, so only
its byte-identical link swaps were taken; its primary submit and the
bg-card button family are left for a pass that can UAT the screen.

3x tsc, eslint, knip, prettier, 689 vitest green; UAT on Notifications,
Messages and the nav rail, clean console in a fresh tab.

frontend/src/components/PlexPlaylistView.tsx

@@ -35,6 +35,7 @@ import { api, type PlexCard } from "../lib/api";
 import { formatRuntime } from "../lib/format";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 import { useConfirm } from "./ConfirmProvider";
+import { Button } from "./ui/Button";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import ViewSwitcher, { type ViewOption } from "./ViewSwitcher";
 
@@ -514,13 +515,10 @@ export default function PlexPlaylistView({
 
   return (
     <div className="mx-auto w-[90%] max-w-[1100px] p-4">
-      <button
-        onClick={onBack}
-        className="glass-card glass-hover mb-4 inline-flex items-center gap-1.5 rounded-lg px-2.5 py-1.5 text-xs"
-      >
+      <Button variant="glass" size="xs" className="mb-4" onClick={onBack}>
         <ArrowLeft className="h-4 w-4" />
         {t("plex.backToLibrary")}
-      </button>
+      </Button>
 
       <div className="glass rounded-2xl p-4 sm:p-6">
         <div className="mb-4 flex flex-wrap items-center gap-3">