Add palladium chain configuration files

Add initial configuration files for Palladium chain including servers, checkpoints, and fallback nodes.
Update constants to use Palladium as default network and rename PalladiumMainnet to Palladium for consistency.

electrum/chains/palladium/checkpoints.json

@@ -0,0 +1,17 @@
+[
+    ["000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f", 0],
+    ["00000000082962e4c2838933cb63507142c1abb748d84b7ddce6bb233d6407e0", 1],
+    ["000000004cc3eca82841f0691e6231b86c3b269e447fa7d6e7221cd42f725390", 16],
+    ["00000000ae75d0169080e9f0ddbcd80827eda623cfe1f4a2b1be6dcd49b916e6", 69],
+    ["000000000000086425f826a2eb60c588aefd3e0783ddeccf0f4f0c985d348e69", 22170],
+    ["00000000000000d66df607146de7d9b423cf97150beb804d22439d199e868ca9", 26619],
+    ["0000000000000017e9e74b9b403b775098905418b1333e9612f510af66746aa7", 28879],
+    ["0000000000000014351dee34029945d5a4dea299ec8843626695c88b084b4d10", 28925],
+    ["000000000000041fddecba51204a679b15ae47fc8aa658ef4ea7b953445d95e5", 50000],
+    ["0000000000000850eba93bbc491f085e2c79c0c30c497292858c72e90cae69a5", 100000],
+    ["000000000000829a0a4cab2f040151766df64edfe8817c565d101ae12b51411a", 142892],
+    ["00000000000003212d753a62f2dec5b696ab22524cc49ba7cdc0d80c45d0eb18", 150000],
+    ["000000000000221a9e16556453fc86308b260d95d80c14bafaf053a09374e7eb", 200000],
+    ["0000000000012553b0303deaf5f2883deb66c901b6848dd03bb4a34f1774e0d0", 250000],
+    ["0000000000013acdf07a4fb988bbe9824c36eb421478a71c8196cf524dcba143", 300000]
+]

electrum/chains/palladium/fallback_lnnodes.json

@@ -0,0 +1 @@
+{}

electrum/chains/palladium/servers.json

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+{
+    "173.212.224.67": {
+        "pruning": "-",
+        "s": "50002",
+        "t": "50001",
+        "version": "1.4.2"
+    },
+    "144.91.120.225": {
+        "pruning": "-",
+        "s": "50002",
+        "t": "50001",
+        "version": "1.4.2"
+    },
+    "66.94.115.80": {
+        "pruning": "-",
+        "s": "50002",
+        "t": "50001",
+        "version": "1.4.2"
+    },
+    "89.117.149.130": {
+        "pruning": "-",
+        "s": "50002",
+        "t": "50001",
+        "version": "1.4.2"
+    }
+}