Keep showing last known values when the bike is out of range

Setup no longer depends on the bike being reachable. The coordinator is
built from an address and resolves the BLEDevice per connect attempt, so
a parked or switched-off bike loads the entry instead of raising
ConfigEntryNotReady and leaving every entity unavailable.

Parser state is persisted through helpers.storage.Store and restored
before the platforms are set up, so entities carry their last values and
the VIN on their first state write. What survives is a whitelist:
battery and EBM counters yes, motor and assist no - a restored speed
reading is indistinguishable from live data on a parked bike. The
connection switch position is restored too, so a restart no longer wakes
a bike the user deliberately disconnected.

Also fixes three defects found while building this:

- Store.async_delay_save debounces rather than throttles, so re-arming on
  every notification postponed the write for as long as the bike stayed
  connected and nothing reached disk except on a clean shutdown.
- establish_connection had no disconnected_callback, so a dropped link
  left _is_connected True: the connectivity sensor lied and the poll
  never retried.
- _connect read self._client back across the 200ms handshake, which a
  concurrent teardown could clear underneath it.

Connecting now starts on the bike's advertisement instead of the next
poll tick, and an unreachable bike says why - distinguishing "switched
off" from "seen only by a passive proxy that cannot connect".

Renames the connection switch to Auto-connect and drops the hardcoded
English name on the connectivity sensor, which had been defeating its
translation key.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FyabWZzd7HoLpyBwEa5Zzh

custom_components/mysmartbike_ble/translations/en.json

@@ -27,10 +27,15 @@
     }
   },
   "entity": {
+    "binary_sensor": {
+      "connected": {
+        "name": "Connected"
+      }
+    },
     "switch": {
       "connection": {
-        "name": "Connection"
+        "name": "Auto-connect"
       }
     }
   }
-}
+}