Add Tokenizer Mode Support for Seshat Search Index

[shinaoka]

element-meta Issue #2883: i18n support for full-text search in encrypted rooms

Summary

This PR adds tokenizer mode support to element-desktop, enabling users to choose between standard (language-based) and N-gram tokenization for the local message search index.

Changes

• Add tokenizerMode option to EventIndex initialization
• Accept tokenizerMode from element-web via IPC
• Support both N-gram and language-based tokenization modes
• Default to language-based tokenization for backward compatibility

Files Changed

• src/@types/matrix-seshat.d.ts - Add TypeScript types for tokenizer options
• src/seshat.ts - Pass tokenizerMode to Seshat, handle IPC from element-web

Dependencies

⚠️ This PR depends on:

• Seshat: PR #150: Add N-gram Tokenizer Mode for CJK Language Support

Seshat PR must be merged and a new npm package version released before this PR can be merged.

After Seshat is released, update package.json to use the new matrix-seshat version (e.g., ^4.1.0).

Merge Order

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

Checklist

• Ensure your code works with manual testing.
• 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)

[dbkr]

This needs the other two PRs to land first (which will need a look from product).