Implement reliability improvements across frontend

- api.js: add exponential backoff retry (3 attempts, 500/1000/2000ms) for
  GET requests on network errors and 5xx responses; mutating methods are
  not retried since they are not idempotent
- api.js: add offline indicator — fixed pill banner appears at bottom of
  page when navigator goes offline, disappears when back online
- style.css: add styles for offline banner and session expiry warning
- auth.js: show amber warning banner below nav when session expires within
  24 hours (with exact hours remaining); dismissible with X button
- auth.js: fix password min-length client-side check from 6 to 10 to
  match the backend
- log.js, flock.js, budget.js: disable submit button during async request
  and re-enable in finally block to prevent double-submits and make loading
  state visible
- dashboard.js: fix chart date labels to use user's configured timezone
  instead of the browser's local timezone

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx/html/css/style.css

@@ -377,3 +377,40 @@ td input[type="date"] {
     border-top: 1px solid var(--border);
     margin: 1.25rem 0;
 }
+
+/* ── Offline banner ───────────────────────────────────────────────────────── */
+#offline-banner {
+    position: fixed;
+    bottom: 1rem;
+    left: 50%;
+    transform: translateX(-50%);
+    background: #333;
+    color: #fff;
+    padding: 0.5rem 1.25rem;
+    border-radius: 99px;
+    font-size: 0.875rem;
+    z-index: 1000;
+    box-shadow: 0 2px 8px rgba(0,0,0,0.3);
+}
+
+/* ── Session expiry warning ───────────────────────────────────────────────── */
+#session-warning {
+    background: #fff3cd;
+    color: #856404;
+    border-bottom: 1px solid #ffc107;
+    padding: 0.5rem 1.5rem;
+    font-size: 0.875rem;
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    gap: 1rem;
+}
+#session-warning button {
+    background: none;
+    border: none;
+    font-size: 1rem;
+    cursor: pointer;
+    color: inherit;
+    padding: 0 0.25rem;
+    line-height: 1;
+}