feat(ui): dual-ecash wallet settings, buy-peer-files, seed backup, assorted fixes

- Tabbed Wallet Settings modal (Cashu + Fedimint) and dual-balance wallet card
- Buy a peer's paid file (ecash / node Lightning / on-chain / external QR)
- Recovery-phrase reveal + backup section; onboarding seed retry resilience
- NetBird HTTPS launch, remote-control two-finger scroll + external-open
- Shared BackButton, single-v version label, mesh Bitcoin header toggles

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

neode-ui/src/style.css

@@ -729,6 +729,36 @@ input[type="radio"]:active + * {
     min-height: 36px;
   }
 
+  /* Transparent "frosted" variant for back buttons — the light counterpart to
+     the solid black .glass-button. Used by the floating mobile back pill so it
+     reads as transparent over content rather than a black slab. */
+  .back-button-glass {
+    position: relative;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    min-height: 44px;
+    background: rgba(255, 255, 255, 0.08);
+    backdrop-filter: blur(24px) saturate(140%);
+    -webkit-backdrop-filter: blur(24px) saturate(140%);
+    border: 1px solid rgba(255, 255, 255, 0.16);
+    box-shadow:
+      0 8px 24px rgba(0, 0, 0, 0.28),
+      inset 0 1px 0 rgba(255, 255, 255, 0.18);
+    color: rgba(255, 255, 255, 0.92);
+    transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1), background-color 0.2s ease, border-color 0.2s ease;
+  }
+
+  .back-button-glass:hover {
+    transform: translateY(-2px);
+    background: rgba(255, 255, 255, 0.14);
+    border-color: rgba(255, 255, 255, 0.26);
+  }
+
+  .back-button-glass:active {
+    transform: translateY(1px);
+  }
+
   /* Glass button color variants */
   .glass-button-warning {
     background: rgba(251, 146, 60, 0.2);