fix: debounce debug log preview filter to keep focus while typing

[Exotic209093]

Summary

• keeps the Debug Log Viewer custom filter input focused while typing
• debounces preview filter application instead of reprocessing the preview on every keystroke
• keeps the live input value separate from the applied filter and cancels stale queued updates

How it works

When the user types in the custom filter box, the textbox value updates immediately, but the expensive preview filtering is only applied after a short debounce. The filter input is no longer disabled during preview filter processing, so focus is not dropped mid-typing.

Test Plan

• Open Logs Viewer (beta)
• Open any log preview
• Type continuously in the Custom Filter field
• Verify the input keeps focus and the cursor does not jump out while typing
• Verify the selected filter template still updates the preview correctly
• Close and reopen a preview to confirm the filter resets cleanly

Closes #1063