feat: add writing tools settings and native spellcheck

[Ipsumlorem]

Summary

• add global writing-tools settings with typed preload/IPC support
• add a dedicated Settings section for writing tools configuration
• enable native spellcheck in the chat composer and wire it to the new settings
• merge the branch forward to current upstream main and resolve chat merge conflicts cleanly

Included in this PR

• global writing-assist config defaults and normalization
• main-process persistence helpers
• preload API surface for writing-assist settings
• Settings UI for spellcheck, autocomplete, and translation preferences
• native spellcheck wiring for the chat input

Not included

• autocomplete behavior
• translation actions
• grammar rewrite
• remote/SSH parity for LLM-backed helpers

Verification

• node typecheck passed
• web typecheck passed
• targeted tests passed (190 tests)

Notes

This PR intentionally ships a vertical slice: settings plus working native spellcheck in chat. Follow-up work will build translation and autocomplete on top of this base.