[Architecture] Prepare migration path from Electron to Tauri

[shadowbipnode]

Refactor browser architecture to reduce Electron coupling and prepare a future Tauri migration.

Goals:

• isolate browser engine abstractions
• isolate IPC layer
• separate UI state from Electron APIs
• reduce renderer/main process coupling
• prepare Rust-compatible backend boundaries
• modularize oversized Electron-specific modules
• abstract filesystem, dialogs and native integrations
• prepare multi-platform WebView compatibility

Long-term target:

• optional future migration from Electron to Tauri
• reduced memory footprint
• improved security model
• native Rust integrations

Notes:

• this is an architectural epic
• migration itself is NOT part of this issue
• focus is preparing the codebase incrementally

[shadowbipnode]

For v0.6 this should remain research/planning only. Do not migrate the app to Tauri in this milestone. The goal is to document boundaries, risks and future migration steps while continuing Electron hardening.