feat(desktop): add annotated diff viewer for session findings

[justn-hyeok]

Problem

Session detail shows findings and markdown, but not an annotated diff view tying findings to file/line context.

Source

docs/DESKTOP_APP_CONSOLIDATION.md lists annotated diff viewer as a web dashboard capability to absorb.

Acceptance Criteria

• Session detail can show file-by-file diff context when available.
• Findings are visually attached to relevant files/lines.
• Missing diff artifacts degrade gracefully.
• Typecheck and smoke pass.