id: DISPLAY-001
title: Underlying Mode Switching
status: backlog
priority: P0
size: M
depends_on:
  - PORTFOLIO-001
  - DATA-004
tags: [ui, display-mode]
summary: Allow the entire app to switch between GLD, GC=F, and XAU display modes.
acceptance_criteria:
  - Settings page has "Display Mode" selector: GLD shares, Gold Ounces, Gold Grams, GC=F contracts
  - In GLD mode: GLD positions show direct values, other positions converted using entry-date GLD backing
  - In Gold mode: All positions converted to equivalent oz/g using entry-date conversions
  - In GC=F mode: GC=F positions show direct values, others converted
  - Overview shows collateral in display mode units
  - Hedge page shows protection in display mode units
  - Backtests show results in display mode units
  - Mode persists per workspace
notes:
  - Conversion uses GLD_ounces_per_share(entry_date) for historical accuracy
  - GC=F contract size is fixed at 100 oz
  - XAU grams to oz conversion: 31.1035 g/oz
  - Mode switching is a display concern, not a data concern
  - Position data remains in original units
implementation_hints:
  - Add `display_mode: "GLD" | "XAU_OZ" | "XAU_G" | "GCF"` to `PortfolioConfig`
  - Add `convert_to_display(position, display_mode, reference_date)` in `app/domain/conversions.py`
  - Reuse existing `gld_ounces_per_share(date)` for historical conversion
  - Overview/hedge/backtests use display conversion layer
  - Settings page saves display_mode preference