Migrate chart generation from Globadge to QuickChart

[whywaita]

Fixes: #97

• Replace Globadge API with QuickChart API for chart image generation
• Globadge API is returning 403 errors, making charts unavailable

Changes

Chart API Migration

• Before: https://api.globadge.com/v1/chartgen/line/time and https://api.globadge.com/v1/chartgen/stacked-area/time
• After: https://quickchart.io/chart/create

Modified Functions

Function	Description
getLineGraph()	Generates line charts for Network I/O and Disk I/O metrics
getStackedAreaGraph()	Generates stacked area charts for CPU, Memory, and Disk Usage metrics

Technical Details

• Uses Chart.js v2 configuration format supported by QuickChart
• Time-series data with type: 'time' scale for X-axis
• Stacked area charts implemented using type: 'line' with fill and stacked: true options
• Theme support (light/dark) preserved via axisColor configuration
• Short URL generation via /chart/create endpoint returns { success: true, url: "..." }

Tests

I tested in https://github.com/whywaita/keex/actions/runs/20159448564 .

[hridujain]

@serkan-ozal any chance you could review this PR soon? Our team is eagerly awaiting a fix for the linked problem #97!