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/lib/useActiveFilters.ts

@@ -11,7 +11,7 @@ export interface ActiveFilter {
 }
 
 /** What the feed looks like with nothing applied — the baseline every "active" check reads against. */
-export const DEFAULT_SIDEBAR_FILTERS: Omit<FeedFilters, "q" | "scope"> = {
+const DEFAULT_SIDEBAR_FILTERS: Omit<FeedFilters, "q" | "scope"> = {
   tags: [],
   tagMode: "or",
   channelIds: [],