feat: Add Han Unification detection in note editor (Stage 1)

[xenonnn4w]

Purpose / Description

Implements Stage 1 of Han Unification issue detection to help users who are learning Japanese (or other CJK languages) identify rendering problems in their flashcards.

Fixes

• Improve UX for resolving 'Han Unification' rendering issues #19431

Approach

Stage 1: Detection & Logging Only

This is the first stage of a multi-stage implementation. Stage 1 focuses on detection and logging without any user-facing UI changes.

1. Detection at Note Edit/Add Time (Not Review Time)
2. Plain Text Detection (Not HTML Parsing)
3. Curated Character List (Not All CJK)
   • Checks for 23 known problematic CJK characters with significant visual differences
   • Reduces false positives compared to checking all 20,000+ CJK Unified Ideographs

How Has This Been Tested?

Learning (optional, can help others)

Han Unification Background:

• Wikipedia: Han Unification - Overview of the Unicode concept
• Unicode CJK Unified Ideographs - Technical details
• W3C: Language Tags in HTML - How lang attributes solve this

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner