Fix: re-add Crab to blacklist, mistakenly dropped in earlier cleanup

Real user hit a stuck "Connecting..." state on the Crab chain on a
live build. Root cause: the ORIGINAL blacklist (before today's
cleanup) correctly listed this chain as "Darwinia Crab" with chain_id
86e49c...c3a - I dropped that entry during the 2026-07-08 blacklist ID
correction, having concluded it "no longer exists upstream" based on
a name search for "darwinia"/"crab" that didn't match because the
chain's actual name field is just "Crab", not "Darwinia Crab" - a
search-term miss, not evidence the chain or its chain_id had changed.
It's been in chains.json all along with the same id and the same dead
RPC (crab-rpc.darwinia.network, confirmed unreachable again live
2026-07-08).

Re-verified against live chains.json before re-adding - it's really
there, and the RPC is really dead.

pezkuwi-overlay/chains/blocked-chains.json

@@ -135,6 +135,10 @@
     {
       "name": "krest",
       "reason": "krest.api.onfinality.io unreachable - confirmed live 2026-07-08"
+    },
+    {
+      "name": "Crab",
+      "reason": "crab-rpc.darwinia.network unreachable - was correctly blacklisted before (as \"Darwinia Crab\") but got dropped by mistake during the 2026-07-08 blacklist ID cleanup, based on a wrong assumption that the chain no longer existed upstream. It does (name is just \"Crab\", not \"Darwinia Crab\" - a name-matching miss, not a chain-ID change). Re-confirmed unreachable live 2026-07-08, and directly confirmed by a real user hitting a stuck \"Connecting\" state on this exact chain."
     }
   ],
   "chain_ids": [
@@ -170,6 +174,7 @@
     "dffb39a66d80b9adb6bdbd7564a9215a1606596062578bf536480de6cc780c2d",
     "dd6d086f75ec041b66e20c4186d327b23c8af244c534a2418de6574e8c041a60",
     "eip155:420420422",
-    "b3dd5ad6a82872b30aabaede8f41dfd4ff6c32ff82f8757d034a45be63cf104c"
+    "b3dd5ad6a82872b30aabaede8f41dfd4ff6c32ff82f8757d034a45be63cf104c",
+    "86e49c195aeae7c5c4a86ced251f1a28c67b3c35d8289c387ede1776cdd88b24"
   ]
 }