[P1][WEB-02] Daily browser features and account profiles

Parent: #1

## Objective
Make Original mode a dependable daily browser rather than a demo webview, while keeping profiles/account state isolated from AI and connector authority.

## Scope
- Tabs/windows, back/forward, history, bookmarks, restore and private sessions.
- Authentication, OAuth handoff, passkeys/WebAuthn, MFA handoff and password-manager interoperability.
- Site permissions, downloads/uploads, media, popups, printing, file choosers and notifications.
- Import from common browsers where legally/technically feasible.
- Profile partitions for cookies/storage/cache and clear account identity in trusted chrome.
- Explicit extension-compatibility policy and feature support matrix.

## Acceptance criteria
- [ ] Declared browser feature matrix passes on every advertised OS.
- [ ] Two profiles/accounts cannot leak cookies, cache, history or authenticated actions across boundaries.
- [ ] Passkey/MFA flows hand back correctly to the authenticated Original view.
- [ ] Download/upload/permission prompts cannot be spoofed by generated workspace UI.
- [ ] Crash/session restore returns to the correct profile and navigation state.
- [ ] Unsupported extension/browser features are disclosed rather than silently degraded.

## Dependencies
- WEB-01
- SEC-03

**First phase:** P1
**Maturity target:** P7
**Workstream:** Conventional browser and compatibility

Start with parent issue #1 and review dependencies WEB-01 and SEC-03 before scoping this workstream. Break the browser feature matrix into independently verifiable areas, beginning with the advertised OS targets and profile isolation requirements. Done means the listed acceptance criteria are met without silent degradation, including authentication handoff, session restore, and prompt boundaries.

Written by the indexing model from the issue text.