Add XIAO Wio-SX1262 overlay; support SX12xx bandwidths (including 62.5 kHz) as default

- boards/seeed/xiao_wio_sx1262: add overlay, .conf, board.yml
- app: support full set of SX12xx bandwidths; default BW set to 62 (62.5 kHz)
- Update Kconfig and prj.conf
- Update lora_modem.h/c for validation and mapping

.claude/settings.local.json

@@ -0,0 +1,17 @@
+{
+  "permissions": {
+    "allow": [
+      "WebSearch",
+      "WebFetch(domain:docs.zephyrproject.org)",
+      "WebFetch(domain:github.com)",
+      "WebFetch(domain:raw.githubusercontent.com)",
+      "WebFetch(domain:www.semtech.com)",
+      "WebFetch(domain:wiki.seeedstudio.com)",
+      "WebFetch(domain:heltec.org)",
+      "WebFetch(domain:www.seeedstudio.com)",
+      "WebFetch(domain:store.rakwireless.com)",
+      "WebFetch(domain:semtech.my.salesforce.com)",
+      "WebFetch(domain:resource.heltec.cn)"
+    ]
+  }
+}