Add column pinning custom theme samples for Grid, Hierarchical Grid, and Tree Grid

[Copilot]

This PR adds three new column pinning samples with custom palette theming, following the same pattern established in PR #927 for cell editing.

What's Added

Created column-pinning-custom-theme samples for:

• Grid: samples/grids/grid/column-pinning-custom-theme/
• Hierarchical Grid: samples/grids/hierarchical-grid/column-pinning-custom-theme/
• Tree Grid: samples/grids/tree-grid/column-pinning-custom-theme/

Key Features

Each sample demonstrates custom palette theming for column pinning with:

Color Palette System

• Organized CSS variables for base colors (white, blue, orange, gold, gray, dark-gray, border-gray)
• Semantic palette variables (primary, secondary, surface, accent, highlight, background, border)
• Secondary color variations for consistent theming

Column Pinning Styling

The samples showcase custom styling of pinned columns using palette variables:

.custom-grid-palette-theme { --ig-grid-pinned-border-width: 5px; --ig-grid-pinned-border-color: var(--palette-accent); --ig-grid-pinned-border-style: double; --ig-grid-cell-active-border-color: var(--palette-accent); }

Visual Design

• Uses dark bootstrap theme for modern appearance
• Gold accent color (#ffcd0f) for pinned column borders
• Double-border style for clear visual distinction
• Interactive grid toolbar with column pinning UI

Implementation Details

Each sample includes:

• Complete React TypeScript implementation
• Custom CSS with palette variables
• Sample data (CustomersDataLocal.json, HierarchicalCustomersData.json, EmployeesFlatData.ts)
• All necessary configuration files (package.json, tsconfig.json, vite.config.js, .eslintrc.js)
• Documentation (ReadMe.md)

All samples have been validated:

• ✅ TypeScript compilation passes
• ✅ Builds successfully with Vite
• ✅ No security vulnerabilities detected

Fixes #930

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create palletes sample for the column pinning topic</issue_title>
<issue_description>Create samples with custom palletes for the column pinning topic . Refer to column pinning samples in this repo and create samples. The samples should look like the ones in this PRs:
Grid: #929
Hgrid and Tgrid: #927</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #933

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