feat(app): preview workspace panes

[jeonghun-jj-lee]

Implements the app-side work for harmoniqs/amicode#940 and its Preview workspace slices.

Includes:

• multi-file retained renderer hosts and recursive pane placement
• edge split, transfer, resize, CSS-only overflow, and inner/outer drag isolation
• neutral floating tab drag proxy with cancellation fade
• per-pane zoom, destination-pane transfer semantics, and focused-pane chrome

Verification:

• 25 Chromium Preview browser contracts
• 1,124 app unit tests
• 380 session-ui tests
• app typecheck and production build

Closes harmoniqs/amicode#945
Refs harmoniqs/amicode#944

[coderabbitai]

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