fix(analytics): apply customizer timestamp format to chart axes (closes #756) (#981)

## Summary

Fixes #756 — the customizer timestamp format setting (ISO/ISO+ms/locale)
and timezone (UTC/local) were not applied to chart X-axis labels,
tooltips, or certain inline timestamps in the analytics pages.

## Changes

### `public/app.js`
- Added `formatChartAxisLabel(date, shortForm)` — a shared helper that
reads the customizer's `timestampFormat` and `timestampTimezone`
preferences and formats dates for chart axes accordingly.
`shortForm=true` returns time-only (for intra-day charts),
`shortForm=false` returns date+time (for multi-day ranges).

### `public/analytics.js`
- `rfXAxisLabels()`: now calls `formatChartAxisLabel()` instead of
hardcoded `toLocaleTimeString()`
- `rfTooltipCircles()`: tooltip timestamps now use
`formatAbsoluteTimestamp()` instead of raw ISO
- Subpath detail first/last seen: now uses `formatAbsoluteTimestamp()`
- Neighbor graph last_seen: now uses `formatAbsoluteTimestamp()`

### `public/node-analytics.js`
- Packet timeline chart labels: now use `formatChartAxisLabel()`
(respects short vs long form based on time range)
- SNR over time chart labels: now use `formatChartAxisLabel()`

## Behavior by setting

| Setting | Chart axis (short) | Chart axis (long) |
|---------|-------------------|-------------------|
| ISO | `14:30` | `05-03 14:30` |
| ISO+ms | `14:30:05` | `05-03 14:30:05` |
| Locale | `2:30 PM` | `May 3, 2:30 PM` |

All respect the UTC/local timezone toggle.

## Testing

- Server builds cleanly (`go build`)
- Served `app.js` contains `formatChartAxisLabel` (verified via curl)
- Graceful fallback: all callsites check `typeof formatChartAxisLabel
=== 'function'` before calling, preserving backward compat if script
load order changes

---------

Co-authored-by: you <you@example.com>

public/node-analytics.js

@@ -170,7 +170,7 @@
       data: {
         labels: tl.map(b => {
           const d = new Date(b.bucket);
-          return currentDays <= 3 ? d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' }) : d.toLocaleDateString([], { month: 'short', day: 'numeric' });
+          return (typeof formatChartAxisLabel === 'function') ? formatChartAxisLabel(d, currentDays <= 3) : (currentDays <= 3 ? d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' }) : d.toLocaleDateString([], { month: 'short', day: 'numeric' }));
         }),
         datasets: [{ label: 'Packets', data: tl.map(b => b.count), backgroundColor: 'rgba(74,158,255,0.5)', borderColor: '#4a9eff', borderWidth: 1 }]
       },
@@ -197,7 +197,7 @@
     const longestObs = Object.values(byObs).sort((a, b) => b.points.length - a.points.length)[0];
     const labels = longestObs ? longestObs.points.map(p => {
       const d = p.x;
-      return d.toLocaleDateString([], { month: 'short', day: 'numeric' }) + ' ' + d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
+      return (typeof formatChartAxisLabel === 'function') ? formatChartAxisLabel(d, false) : d.toLocaleDateString([], { month: 'short', day: 'numeric' }) + ' ' + d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
     }) : [];
     const c = new Chart(ctx, {
       type: 'line',