docs: document unit conversion implementation

- Update implementation-plan.md: mark unit conversion complete
- Update HANDOFF.md: add session notes for 2026-05-24 unit conversion
- Update README.md: list Unit Conversion as a feature

docs/HANDOFF.md

@@ -293,6 +293,29 @@ Trust the tests. Trust the live runs. Don't trust prose claims that something is
 
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+## New session: 2026-05-24
+
+### Unit conversion implementation
+Completed implementation of recipe-to-grocery unit conversion to make cost estimates accurate.
+
+**Files added:**
+- `backend/app/utils/units.py` — `UnitConverter` class
+  - Normalization: maps synonyms to canonical units (e.g. "TBS" → "tbsp", "pounds" → "lb")
+  - Within-family linear conversion: lb↔oz↔g, cup↔tbsp↔tsp, dozen↔ea
+  - Cross-family via density tables for ~30 canonical ingredients (e.g. rice cup→lb via 185g/cup / 453.592g/lb)
+  - Fallback to dimensionless qty when conversion is impossible (preserves monotonic ranking signal)
+
+**Files modified:**
+- `backend/app/services/planner/cost.py` — multiplies `current_price` by `convert_qty(qty, recipe_unit, grocery_unit, ingredient_name)`
+- `backend/app/services/planner/generate.py` — `_load_match_index` now joins `Ingredient` table and returns `ingredient_name` + `grocery_unit` for each match
+- `backend/app/services/orchestrator/steps.py` — both email cost block and shopping-list total now use unit conversion
+- `backend/tests/test_planner_cost.py` — updated fixture to include new fields
+- `backend/tests/test_units.py` — 19 tests covering normalization, within-family, density, and fallback
+
+**Test results:** `test_units.py` 19/19 pass; planner cost/score/select 36 passed.
+
+---
+
 ## New session: 2026-05-23
 
 ### Context