fix: improve code block visibility in dark mode

[AlexAndBear]

Description

This bugfix improves code readability in dark mode by increasing contrast for editor code backgrounds.

Changes made:

• Updated pre code block background token from var(--oc-role-surface-container) to var(--oc-role-surface-container-high)
• Updated inline code background token from var(--oc-role-surface-container) to var(--oc-role-surface-container-high)

File changed:

• packages/web-pkg/src/editor/styles/text-editor.css

Related Issue

• Fixes #<issue_number>

How Has This Been Tested?

• test environment: Local development environment
• test case 1: Opened text editor in dark mode and verified improved code block contrast
• test case 2: Verified inline code background is consistently more visible in dark mode
• test case 3: Confirmed no visual regression in editor code styling structure

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)