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/js/dashboard.js

@@ -1,7 +1,7 @@
 let eggChart = null;
 
 function buildChart(eggs) {
-    const today  = new Date();
+    const tz = (typeof Auth !== 'undefined' && Auth.getUser()?.timezone) || 'UTC';
     const labels = [];
     const data   = [];
 
@@ -9,14 +9,12 @@ function buildChart(eggs) {
     const eggMap = {};
     eggs.forEach(e => { eggMap[e.date] = e.eggs; });
 
-    // Generate the last 30 days in chronological order
+    // Generate the last 30 days in the user's configured timezone
     for (let i = 29; i >= 0; i--) {
-        const d = new Date(today);
+        const d = new Date();
         d.setDate(d.getDate() - i);
-        const y = d.getFullYear();
-        const mo = String(d.getMonth() + 1).padStart(2, '0');
-        const dy = String(d.getDate()).padStart(2, '0');
-        const dateStr = `${y}-${mo}-${dy}`;
+        const dateStr = d.toLocaleDateString('en-CA', { timeZone: tz });
+        const [, mo, dy] = dateStr.split('-');
         labels.push(`${mo}/${dy}`);
         data.push(eggMap[dateStr] ?? 0);
     }