feat(desktop): dock the built-in browser beside chat

[devswha]

What this changes

The macOS built-in browser opens in a resizable panel beside the conversation. Its chrome has a page-title pill, a separate rounded address row, matching SVG navigation icons and subtle loading feedback. It follows the app's active theme and language.

The expand control gives the browser the full window and restores the previous split width. Closing the browser restores the conversation without losing its draft. The original native browser ownership, document bindings, profile and restart fences remain in place. WebView-scoped permissions keep the remote page separate from app controls; the appearance reader is available only to the bundled toolbar.

Closes #110.

Light-theme screenshot

Why

A separate browser window interrupts supervision of the conversation. The integrated panel keeps the page alongside chat, with familiar browser navigation and a visual hierarchy based on the supplied Codex reference.

Verification

• npm run verify passed.
• Final native formatting and locked Tauri tests passed: 381 native tests, 6 existing opt-in cases ignored, and 11 build-binding tests.
• Ten toolbar DOM tests cover title escaping, icons, appearance, address editing, expanded state and divider input.
• The verified macOS payload and ad-hoc app build passed. A copy launched outside the checkout and opened the redesigned browser.
• On macOS 26.6.2 / arm64, 17 authenticated REST-to-native checks passed, including IPC denial for app controls and appearance, unchanged agent state, ownership, DOM actions, navigation, stale bindings and confirmed close.
• Isolated GUI checks passed for light/dark matching, expand/restore with split width and draft preservation, close while expanded, blocked-address feedback, normal navigation and quit.

Local ad-hoc validation only; no production installation, notarization or release publication.

---

• I have signed the Contributor License Agreement, or I am the project owner.
• npm run verify passes, or I have said below which gate fails and why.