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Description

Introduces a unified Preview Panel on the session page, replacing the previous single-purpose file preview with a tabbed surface that supports browser, file, canvas, terminal, and review content types.

Preview panel & layout

• Adds PreviewPanel with a scrollable tab strip, chooser flow for new tabs, and per-tab content routing (BrowserTab, FileTab, CanvasTab, TerminalTab, ReviewTab, ChooserTab).
• Refactors the session layout to a resizable chat/preview split with open/close animation; browser webviews are held until the panel animation settles so pages do not flash over the chat mid-transition.
• Extends pageTabStore with per-project preview state (sessionPreviewByProject), tab lifecycle helpers, and openBrowserPreview() for opening or reusing browser tabs by URL.

In-session embedded browser (desktop)

• Adds PreviewBrowserLayer, an always-mounted Electron <webview> layer that positions guests over the active browser tab via previewBrowserViewport, parks them offscreen when hidden, and evicts idle project guests after 10 minutes to reclaim renderer processes.
• Adds browser chrome (back/forward/refresh, address bar, open in system browser) and a webviewRegistry for navigation control without reparenting guests.
• Adds browserUrl.ts for URL normalization and canonicalization.

Integration & security

• Links in chat markdown and user messages open in the session preview browser on desktop instead of leaving the app; web host falls back to window.open.
• Hardens Electron main-process webview attachment: strips unsafe guest preferences, blocks non-HTTP(S) sources, and routes window.open / target=_blank into the same guest instead of spawning popups.
• Refactors FilePreview and folder flows to support embedded mode inside preview tabs.

Tests

• Adds unit coverage for preview panel routing, browser layer viewport/guest behavior, URL helpers, and preview store tab management (30 tests).

Testing Evidence (REQUIRED)

Unit tests (all passing):

✓ test/unit/lib/browserUrl.test.ts (5 tests)
✓ test/unit/store/pageTabStore.preview.test.ts (12 tests)
✓ test/unit/components/Session/PreviewPanel.test.tsx (7 tests)
✓ test/unit/components/Session/PreviewBrowserLayer.test.tsx (6 tests)

Test Files  4 passed (4)
Tests       30 passed (30)

Manual verification (desktop):

• Open a project session and toggle the preview panel from the header.

• Add a browser tab from the chooser; navigate via the address bar and back/forward controls.

• Click an https:// link in a chat message; confirm it opens in the in-session browser.

• Open a file preview tab; confirm file content renders in the embedded tab shell.

• Resize the chat/preview split and close/reopen the panel; confirm the browser tab retains its page.

• Switch between projects; confirm preview state is scoped per project without cross-project flash.

• I have included human-verified testing evidence in this PR.

• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).

• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline