fix: remove pasted text/background colors in Tiptap editor

[AlexAndBear]

Description

When users paste content from websites or emails into Tiptap, inline color / background-color styles can make text unreadable in dark theme (for example black text on dark background).

This change strips pasted text and background colors while preserving other inline styles.

Downsides / trade-offs:

• Intentional source colors and highlights are not preserved on paste.
• Legacy HTML color attributes (color, bgcolor) are removed as well, which can reduce visual fidelity for copied legacy content.

Related Issue

• Fixes <issue_link>

How Has This Been Tested?

• test environment: local macOS dev environment, pnpm + vitest
• test case 1: pnpm --config.confirmModulesPurge=false test:unit --run packages/web-pkg/tests/unit/editor/helpers/paste.spec.ts
• test case 2: pnpm --config.confirmModulesPurge=false test:unit --run packages/web-pkg/tests/unit/editor/composables/useTextEditor.spec.ts
• test case 3: pnpm --config.confirmModulesPurge=false test:unit --run packages/web-pkg/tests/unit/editor/helpers/paste.spec.ts packages/web-pkg/tests/unit/editor/composables/useTextEditor.spec.ts
• ...

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)