Fix LNbits 1.4 compatibility: add null guards for g.user.wallets

LNbits 1.4 changed g.user initialization (PR #3615), moving it from
windowMixin to base.html. This means g.user can be null during initial
Vue template evaluation.

- Use optional chaining g.user?.wallets || [] in template
- Add null guard before accessing this.g.user.wallets in JS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

templates/satmachineadmin/index.html

@@ -547,17 +547,17 @@
         <q-select
           filled
           dense
-          :options="g.user.wallets"
+          :options="g.user?.wallets || []"
           v-model="configDialog.data.selectedWallet"
           label="Source Wallet for DCA Distributions *"
           option-label="name"
           hint="Wallet that holds Bitcoin for distribution to DCA clients"
         ></q-select>
-        
+
         <q-select
           filled
           dense
-          :options="g.user.wallets"
+          :options="g.user?.wallets || []"
           v-model="configDialog.data.selectedCommissionWallet"
           label="Commission Wallet (Optional)"
           option-label="name"