refactor: Replace element ui icons with inhouse SVGs

[KapilSachdev]

Unified SVG Icon System

Problem

echarts-examples currently manages SVG icons using two approaches:

• element-ui CSS class-based icons (el-icon-*)
• Hand-crafted inline SVG elements and standalone SVG files (e.g. sidebar chart list)

<!-- element-ui (Vue 2) -->
<i class="el-icon-edit"></i>
<el-button type="primary" icon="el-icon-edit">Edit</el-button>

As element-ui is Vue 2 only, migrating to element-plus drops the class-based icon API entirely (part of the larger Vue 3 migration #124 ).

<!-- element-plus (Vue 3) -->
<el-icon><Edit /></el-icon>
<Edit />

Rather than adopting element-plus icons and adding yet another icon library, this is a good moment to consolidate.

Why Not an Icon Library?

The project uses several chart-specific icons that have no equivalent in standard icon libraries (Font Awesome, etc.). Using element-plus icons or any other external library would mean maintaining the current hybrid approach.

Solution

• Consolidate all icons into hand-crafted SVGs
• Additionally built into a single SVG sprite file.

What This PR Does

• Removes dependency on element-ui class-based icons
• Adds new SVG icons to keep feature parity for removed icons
• Introduces SVG sprite build
• Updates all icon usages across the project to use the new sprite approach