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.

.gitignore

@@ -16,6 +16,13 @@ node_modules/
 frontend/dist/
 *.log
 
+# Playwright E2E (browsers install globally; these are per-run artifacts + the auth state)
+frontend/test-results/
+frontend/playwright-report/
+frontend/blob-report/
+frontend/playwright/.cache/
+frontend/e2e/.auth/
+
 # Data / dumps
 *.dump
 *.sql.gz