v0.4: background reads, JS-state login signals, blind sleeps dropped

get_or_open_tab(activate=True) — added, not replaced. Read paths pass
activate=False; post flows keep the foreground for dialogs and file inputs.

check_login moved off DOM signals onto session state: fb c_user, ig/threads
ds_user_id, twitter twid, yt ytcfg.LOGGED_IN. Three platforms were reporting
OUT on a signed-in account because their composer never mounts in a hidden
tab. The fixed sleep(4)/sleep(5) before each check goes with them — every
signal reads present the moment navigate() returns.

Unreadable cookies raise instead of returning False: "cannot tell" is not
"signed out". tiktok untouched and still activated — it reported
visibilityState "visible" from a sibling-activated tab, so it has never been
read from a genuine background.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

platforms/youtube.py

@@ -93,14 +93,21 @@ async def _wait_for_editor_empty(tab, timeout=30.0):
 
 
 async def check_login(cdp_client):
-    tab, reused = await cdp_client.get_or_open_tab(S["url"], "youtube.com")
+    tab, reused = await cdp_client.get_or_open_tab(S["url"], "youtube.com", activate=False)
     if reused:
         await tab.eval("window.onbeforeunload = null")
         await tab.navigate(S["url"])
-    await asyncio.sleep(5)
     if await tab.exists(S["login_marker"]):
         return False
-    return await tab.eval(
-        "!!document.querySelector('#avatar-btn, button#avatar-btn,"
-        " ytd-topbar-menu-button-renderer #avatar-btn')"
+    # JS state, not the masthead DOM: #avatar-btn needs a fully rendered
+    # foreground tab and reads absent for a signed-in account otherwise.
+    # ytcfg is set by the page's own bootstrap and does not depend on render.
+    state = await tab.eval(
+        "(() => { try { return String(ytcfg.get('LOGGED_IN')); }"
+        " catch (e) { return 'unavailable'; } })()"
     )
+    if state == "true":
+        return True
+    if state == "false":
+        return False
+    raise CDPError("yt: ytcfg.LOGGED_IN unavailable — cannot tell signed-in from signed-out")