Workspace / profile switcher (multi-Google-account support)

[GyaneshSamanta]

Problem

Today the app uses a single persist:notebooklm session partition, so only one Google account is logged in. Users with personal + work accounts have to log out and back in.

Proposed approach

• Add a "Profile" dropdown in settings or title bar.
• Each profile uses a distinct session partition string (e.g. persist:notebooklm-work, persist:notebooklm-personal).
• Switching profiles reloads the webviews against the new partition. No data crosses profiles.

Files to touch

• src/settings.js — new `profiles` array + `activeProfile` key
• src/main.js — pass partition string through to renderer
• src/index.html — webview `partition` attribute needs to be set dynamically (may need to recreate webviews)
• src/renderer.js — profile switcher UI

Acceptance criteria

• Add / rename / delete profiles in settings.
• Switching profiles fully isolates session data.
• Persists across restarts.