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/knip.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://unpkg.com/knip@6/schema.json",
+  "playwright": {
+    "config": ["playwright.config.ts"],
+    "entry": ["e2e/**/*.@(spec|setup).ts"]
+  }
+}