fix: toolbar buttons disappear on window resize

[benjaminfrueh]

Problem

Custom toolbar buttons disappear on toolbar re-render (e.g. when Excalidraw changes to mobile).
The hooks used reference guards to prevent duplicate button insertion. When Excalidraw rebuilt the toolbar these still held stale references to removed buttons, blocking re-insertion. Additionally, each hook implemented its own rendering logic.

Solution

• Created a centralized configurable ToolbarButton component to handle the custom toolbar button rendering
• Added a MutationObserver to watch for Excalidraw class changes and re-render buttons automatically
• Refactored all toolbar button hooks (useEmojiPicker, useSmartPicker, useTableInsertion, useAssistant, useComment) to use the centralized component

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[juliusknorr]

Rebased and resolved the conflicts

[benjaminfrueh]

This PR will conflict with #902 and should be tested and updated for the insert table button after #902 is merged.