Add Block Explorer config, log TLS handshakes at notice level

Two related polish items:

1. Optional custom block explorer. New union under Advanced config
   ("Block Explorer") defaulting to "mempool.space". Picking
   "Custom URL" reveals a single text field where the user can
   point Kamado at their own mempool instance — useful for users
   running mempool as a sibling StartOS service or on the same LAN.
   The entrypoint reads .advanced.mempool-explorer.{type,url} and
   exports MEMPOOL_BASE_URL when type=custom; otherwise leaves it
   empty and the UI keeps its mempool.space defaults.

2. stunnel debug level 4 (warning) was hiding successful TLS
   handshakes — only failures showed up in the service logs, which
   made it hard to confirm "yes, my miner did connect over TLS"
   without going looking at netstat. Bump to level 5 (notice) so
   each successful handshake produces an "accepted connection from
   <ip>" / "connected from <ip>" line. Errors stay visible at level
   3, so the only thing this changes is making the happy path
   observable.

scripts/procedures/getConfig.ts

@@ -179,6 +179,40 @@ export const getConfig: T.ExpectedExports.getConfig = compat.getConfig({
         },
         "default": "info",
       },
+      "mempool-explorer": {
+        "type": "union",
+        "name": "Block Explorer",
+        "description":
+          "Where Kamado's UI links should send you when you click a block hash or a user's BTC address. Defaults to mempool.space (the public instance). Switch to 'Custom' to point Kamado at a self-hosted mempool.space mirror — useful if you run mempool as another StartOS service or on the same network.",
+        "tag": {
+          "id": "type",
+          "name": "Source",
+          "description": "Public mempool.space, or your own instance.",
+          "variant-names": {
+            "default": "mempool.space (default)",
+            "custom": "Custom URL",
+          },
+        },
+        "default": "default",
+        "variants": {
+          "default": {},
+          "custom": {
+            "url": {
+              "type": "string",
+              "name": "Mempool URL",
+              "description":
+                "Base URL of your mempool instance, e.g. https://mempool.example.com. Kamado will append /address/<addr> and /block/<hash> to it, so the instance must follow the standard mempool.space URL layout.",
+              "nullable": false,
+              "default": "",
+              "pattern": "^https?://[^\\s]+$",
+              "pattern-description":
+                "Must be an http:// or https:// URL with no whitespace.",
+              "masked": false,
+              "copyable": false,
+            },
+          },
+        },
+      },
     },
   },
 });