server: guard against None snapshots crashing the accept loop

If extract_all() returns None (Dashboard init failure), appending it to
store["records"] would cause _accept_loop to crash with AttributeError on
the next client connection.  Since only OSError was caught, the thread
died permanently — no further clients could join push_clients, so live
broadcasts reached nobody.

- collect.py: skip None snapshots instead of appending them
- network.py: filter None records in _accept_loop; catch all exceptions
  during history send (not just OSError) so the thread survives
- storage_helpers.py: filter None records when loading from disk

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/network.py

@@ -24,15 +24,17 @@ def _accept_loop(
                 preloaded = store_ref.get("preloaded", [])
 
                 if preloaded:
-                    seen_ts = {r.get("ts") for r in preloaded}
-                    extra   = [r for r in in_mem if r.get("ts") not in seen_ts]
-                    records = sorted(preloaded + extra, key=lambda r: r.get("ts", ""))
+                    seen_ts = {r.get("ts") for r in preloaded if r is not None}
+                    extra   = [r for r in in_mem if r is not None and r.get("ts") not in seen_ts]
+                    records = sorted(preloaded + extra, key=lambda r: r.get("ts", "") if r is not None else "")
                 else:
-                    records = in_mem
+                    records = [r for r in in_mem if r is not None]
 
                 try:
                     prev: dict = {}
                     for record in records:
+                        if record is None:
+                            continue
                         diff = jay_diff_full(prev, record, combine_upd_add=True)
                         prev = record
                         conn.sendall((json.dumps(diff, default=str) + "\n").encode())
@@ -42,6 +44,10 @@ def _accept_loop(
                     log.debug("Failed to send history to %s", addr)
                     conn.close()
                     continue
+                except Exception:
+                    log.exception("Error sending history to %s — dropping client", addr)
+                    conn.close()
+                    continue
                 clients.append(conn)
         except OSError:
             break