feat(desktop): add discussion/debate viewer

[justn-hyeok]

Problem

Desktop currently shows discussion/evidence counts, but not the actual discussion/debate content.

Source

docs/DESKTOP_APP_CONSOLIDATION.md lists discussion/debate viewer as a capability to absorb from the old dashboard.

Acceptance Criteria

• Session detail can display discussion/debate artifacts when present.
• Empty state is explicit when no discussions exist.
• Content is readable and scrollable with keyboard access.
• Typecheck and smoke pass.