Add Tokenizer Mode Support for Message Search

[shinaoka]

Summary

This PR adds a user-facing setting to choose the tokenizer mode for local message search. N-gram tokenization is essential for languages without clear word boundaries such as Japanese, Chinese, and Korean (CJK languages).

Changes

• Add tokenizer mode setting (Standard/Japanese) in Message Search preferences
• Pass tokenizer mode to Seshat via element-desktop IPC
• Add confirmation dialog when changing tokenizer mode (requires reindex)

Dependencies

⚠️ This PR depends on:

• Seshat: PR #150: Add N-gram Tokenizer Mode for CJK Language Support
• element-desktop: PR #2753: Add Tokenizer Mode Support for Seshat Search Index

Both upstream PRs must be merged before this PR can be merged.

Merge Order

1. Seshat PR #150  →  npm release (e.g., [email protected])
2. element-desktop PR  →  update matrix-seshat dependency
3. element-web (this PR)  →  UI for tokenizer mode selection

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[t3chguy]

@shinaoka seems like you have lint & insufficient test coverage issues

[shinaoka]

I have managed to fix the CI errors!

[t3chguy]

Blocked on matrix-org/seshat#150

[langleyd]

Thanks for the contribution @shinaoka , this seems like a very valuable one.
The product team need a bit of time to consider how we should handle this mode in the UX.
It'll be next week before we have a response.