feat(ui): shared <Button> primitive + migrate ShareDialog/VideoEditor/ChannelPage proofs

Introduce frontend/src/components/ui/Button.tsx — the one action-button primitive
(variants primary|danger|outline|ghost|glass|danger-outline; sizes xs|sm|md|lg|icon;
icon/iconOnly/label/loading/fullWidth/forwardRef). No focus-visible (already centralized
in index.css @layer base). Migrate the cleanest proof sites off btnCls, and flip the
channel Subscribe button to the outline variant so it matches Full history (the user's ask).

frontend/src/components/ChannelPage.tsx

@@ -34,6 +34,7 @@ import type { FeedView } from "../lib/feedView";
 import { channelYouTubeUrl, formatViews } from "../lib/format";
 import { ChannelTagsControl } from "./ChannelTags";
 import { useChannelTagActions } from "../lib/useChannelTagActions";
+import { Button } from "./ui/Button";
 
 // A dedicated channel page: an "About"-style header (banner, avatar, stats, subscribe) over the
 // channel's videos (the catalog filtered to this channel). For an un-subscribed channel it
@@ -441,21 +442,18 @@ export default function ChannelPage({
                 it) so a large channel's paced tail reads as visible progress rather than a stuck
                 spinner. */}
               {ch?.subscribed && !ch.backfill_done && !me.is_demo && (
-                <button
+                <Button
+                  variant="outline"
                   onClick={() => !ch.deep_requested && deepBackfill.mutate()}
                   disabled={ch.deep_requested || deepBackfill.isPending}
+                  loading={ch.deep_requested}
+                  icon={<History className="w-4 h-4" />}
                   title={
                     ch.deep_requested
                       ? t("channel.fullHistoryLoadingHint")
                       : t("channel.fullHistory")
                   }
-                  className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm border border-border text-muted hover:text-accent hover:border-accent disabled:opacity-60 disabled:hover:text-muted disabled:hover:border-border transition"
                 >
-                  {ch.deep_requested ? (
-                    <Loader2 className="w-4 h-4 animate-spin" />
-                  ) : (
-                    <History className="w-4 h-4" />
-                  )}
                   <span className="hidden sm:inline whitespace-nowrap">
                     {!ch.deep_requested
                       ? t("channel.fullHistory")
@@ -466,7 +464,7 @@ export default function ChannelPage({
                           })
                         : t("channel.fullHistoryLoading")}
                   </span>
-                </button>
+                </Button>
               )}
               {!me.is_demo && (
                 <button
@@ -504,24 +502,24 @@ export default function ChannelPage({
               {!me.is_demo &&
                 !ch?.blocked &&
                 (ch?.subscribed ? (
-                  <button
+                  <Button
+                    variant="outline"
                     onClick={onUnsubscribe}
                     disabled={unsubscribe.isPending}
                     title={t("channel.unsubscribe")}
-                    className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm border border-border text-muted hover:text-fg disabled:opacity-50"
+                    icon={<Check className="w-4 h-4" />}
                   >
-                    <Check className="w-4 h-4" />
                     {t("channel.subscribedState")}
-                  </button>
+                  </Button>
                 ) : (
-                  <button
+                  <Button
+                    variant="outline"
                     onClick={() => subscribe.mutate()}
                     disabled={subscribe.isPending}
-                    className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm btn-primary disabled:opacity-50"
+                    icon={<Plus className="w-4 h-4" />}
                   >
-                    <Plus className="w-4 h-4" />
                     {t("channel.subscribe")}
-                  </button>
+                  </Button>
                 ))}
               {/* Tagging is a subscription-scoped action (attach_tag → _user_subscription 404s
                   otherwise), and personal tags are for your subscriptions — so only offer it on a