fix(status): /status blocked the event loop and outran its own client timeout

The backend logged 200 OK for polls the client had already timed out on.
is_running() waited 2s for an Ollama that ships OFF (now 0.5s), is_available()
spawned 'ollama --version' every call and that command blocks ~5s when Ollama is
wedged (now cached), and both ran synchronously inside an async def, stalling the
event loop on every poll while the UI polls continuously (now to_thread).

Measured with Ollama's port blackholed: /status 5.89s -> 0.64s, concurrent GET /
stalled -> 0.06s. Poll timeout in nexus_window.py raised 2s -> 5s for margin.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bin/nexus_window.py

@@ -60,7 +60,11 @@ def _wait_for_backend(timeout: float = 40.0) -> bool:
     deadline = time.time() + timeout
     while time.time() < deadline:
         try:
-            with urllib.request.urlopen(status_url, timeout=2) as r:
+            # 5s, not 2: /status probes Ollama, and a wedged Ollama made it
+            # slower than a 2s ceiling - the backend was up and answering 200
+            # while this loop timed out on every attempt and declared it dead.
+            # The server side is fixed too; this is the margin.
+            with urllib.request.urlopen(status_url, timeout=5) as r:
                 if r.status == 200:
                     return True
         except Exception: