Tiptap RTE: Optimize umb-input-tiptap initialization and rendering

[leekelleher]

Description

Following on from PR #21068, I asked Claude to review the "input-tiptap.element.ts" file to see if there were any performance or optimization recommendations that could be made. It came up with...

• Cache toolbar and statusbar emptiness checks to avoid .flat() calls on every render
• Pre-compute extension styles during initialization instead of processing on every render

Test plan

• Verify RTE toolbar renders correctly when configured
• Verify RTE statusbar renders correctly when configured
• Verify RTE without toolbar/statusbar still functions
• Verify custom stylesheets are applied to RTE content

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR optimizes the Tiptap RTE component's initialization and rendering performance through three targeted improvements: simplifying the sequential extension loading pattern, caching toolbar/statusbar visibility checks to avoid repeated array operations, and pre-computing extension styles during initialization rather than on every render.

Key Changes:

• Replaced redundant Promise.all wrapper with clear sequential awaits in firstUpdated()
• Added cached #hasToolbar and #hasStatusbar flags to avoid .flat() operations on every render
• Consolidated extension styles into a single CSSResultGroup computed once during initialization