refactor: split main/index.js IPC handlers by domain

[krwg]

Summary

main/index.js (~1965 lines) accumulates IPC for calls, files, mesh, settings, etc. Extract domain modules and keep index.js for window/tray/lifecycle + registering those modules.

Suggested layout

• main/ipc/calls.js
• main/ipc/files.js
• main/ipc/mesh.js
• main/ipc/settings.js
• (add more as natural seams appear)

Acceptance

• IPC handlers live under domain modules
• index.js wires BrowserWindow / tray / app lifecycle and imports registrars
• No intentional IPC contract break (preload API stable)