chore(ui): extract shared inputCls + btnCls to ui/form.tsx

The standard input style was copy-pasted verbatim into 5 components
(DownloadCenter/DownloadDialog/ProfileEditor/ShareDialog/VideoEditor) and the
text-button style into 2 (ShareDialog/VideoEditor). Export both from ui/form.tsx
and import them. Byte-identical strings → no visual change.

The settings-family panels (Settings/Config/Setup/Welcome) use a DIFFERENT input
style (bg-card/rounded-xl/focus:border-accent); unifying the two is a design call
left to the glass-consistency epic, not this DRY extract.

frontend/src/components/ShareDialog.tsx

@@ -7,10 +7,7 @@ import Modal from "./Modal";
 import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";
 import { api, type DownloadJob, type ShareLink } from "../lib/api";
-
-const inputCls =
-  "w-full rounded-lg bg-surface border border-border px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-accent/40";
-const btn = "px-3 py-1.5 rounded-lg text-sm font-medium transition disabled:opacity-50";
+import { inputCls, btnCls } from "./ui/form";
 const EXPIRY_DAYS = [null, 1, 7, 30] as const;
 
 // --- share with a registered user (ACL) -----------------------------------------------------
@@ -221,10 +218,10 @@ function LinkShare({ job }: { job: DownloadJob }) {
             autoComplete="new-password"
           />
           <div className="flex justify-end gap-2">
-            <button onClick={() => setCreating(false)} className={clsx(btn, "text-muted hover:text-fg hover:bg-surface")}>
+            <button onClick={() => setCreating(false)} className={clsx(btnCls, "text-muted hover:text-fg hover:bg-surface")}>
               {t("common.cancel")}
             </button>
-            <button onClick={() => create.mutate()} disabled={create.isPending} className={clsx(btn, "bg-accent text-accent-fg hover:opacity-90")}>
+            <button onClick={() => create.mutate()} disabled={create.isPending} className={clsx(btnCls, "bg-accent text-accent-fg hover:opacity-90")}>
               {t("downloads.share.createLink")}
             </button>
           </div>
@@ -232,7 +229,7 @@ function LinkShare({ job }: { job: DownloadJob }) {
       ) : (
         <button
           onClick={() => setCreating(true)}
-          className={clsx(btn, "mt-2 border border-border text-muted hover:text-fg hover:border-muted inline-flex items-center gap-1.5")}
+          className={clsx(btnCls, "mt-2 border border-border text-muted hover:text-fg hover:border-muted inline-flex items-center gap-1.5")}
         >
           <Link2 className="w-4 h-4" /> {t("downloads.share.newLink")}
         </button>