Make entire UI mobile-friendly with responsive breakpoints

Add media queries at 480px/768px across all components: single-column
dashboard cards on phones, progressive table column hiding, smaller
typography, reduced padding, full-width toggle buttons, and compact
header. Desktop layout unchanged.

ui/src/lib/MinersTable.svelte

@@ -278,4 +278,28 @@
   tr.idle td {
     color: var(--fg-dim);
   }
+
+  /* Mobile: hide less critical columns */
+  @media (max-width: 768px) {
+    table :global(th:nth-child(2)),
+    table :global(td:nth-child(2)),
+    table :global(th:nth-child(5)),
+    table :global(td:nth-child(5)),
+    table :global(th:nth-child(6)),
+    table :global(td:nth-child(6)) {
+      display: none;
+    }
+  }
+  @media (max-width: 480px) {
+    table :global(th:nth-child(4)),
+    table :global(td:nth-child(4)),
+    table :global(th:nth-child(7)),
+    table :global(td:nth-child(7)) {
+      display: none;
+    }
+    .section-head {
+      flex-direction: column;
+      gap: 0.25em;
+    }
+  }
 </style>