test(e2e): golden-master Playwright net for the Platform refactor

Locks the five flows the refactor must not break (feed, channel, player,
settings, nav) so later sprints can prove they changed nothing. Adds a thin
data-testid layer and a dedicated seeded non-demo account; the suite runs on
the baked :8080 stack (authoritative) or :5173. No product-behaviour change.

frontend/src/components/ui/DraftSaveBar.tsx

@@ -44,8 +44,8 @@ export function DraftSaveBar({
   // mask) scopes this z-40 and fades the bar's bottom edge. The inline variant is in normal flow
   // and stays put.
   const bar = (
-    <div className={wrap}>
-      <span className="text-sm text-muted">
+    <div data-testid="draft-save-bar" className={wrap}>
+      <span data-testid="draft-save-status" data-state={state} className="text-sm text-muted">
         {state === "saved" ? (
           <span className="text-emerald-500 inline-flex items-center gap-1.5">
             <Check className="w-4 h-4" />
@@ -59,6 +59,7 @@ export function DraftSaveBar({
       </span>
       <div className="flex items-center gap-2">
         <button
+          data-testid="draft-discard"
           onClick={onDiscard}
           disabled={saving || !dirty}
           className="glass-card glass-hover flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm disabled:opacity-40 transition"
@@ -67,6 +68,7 @@ export function DraftSaveBar({
           {labels.discard}
         </button>
         <button
+          data-testid="draft-save"
           onClick={onSave}
           disabled={saving || !dirty}
           className="flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm bg-accent text-accent-fg font-medium shadow-md shadow-accent/20 disabled:opacity-40 transition"