Add design spec for transaction account $/% toggle

docs/superpowers/specs/2026-05-20-transaction-amount-mode-toggle-design.md

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+# Transaction Account Amount Mode Toggle - Design Spec
+
+**Date:** 2026-05-20
+**Feature:** Global $/% Toggle for Transaction Accounts (Manual Action)
+
+## Overview
+
+In the transaction edit modal's "manual" action view, replace the static "$" column header with a sliding toggle that allows users to switch between viewing amounts as dollar values or percentages. When toggled, the entire account grid re-renders via HTMX with converted values. Percentages are multiplied by 100 (e.g., $200 on a $200 transaction → 100%). When switching back to dollars, use `spread-cents` to ensure accurate cent distribution.
+
+## Motivation
+
+The cljs master version supports per-row $/% toggles. Users want this capability in the SSR version, but with a single global toggle in the table header for simplicity and consistency with the bulk coding interface.
+
+## Schema Changes
+
+### Form State
+
+Add `amount-mode` to the edit form's step params:
+
+```clojure
+[:amount-mode [:enum "$" "%"] {:default "$"}]
+```
+
+Stored in `multi-form-state` alongside existing transaction data. Not persisted to Datomic—purely a UI preference.
+
+## UI Design
+
+### Table Header
+
+Replace the static `"$"` header cell (line ~739 in `edit.clj`) with a radio toggle:
+
+```clojure
+(com/radio-card {:options [{:value "$" :content "$"}
+                           {:value "%" :content "%"}]
+                 :value (or amount-mode "$")
+                 :name "step-params[amount-mode]"
+                 :hx-post (bidi/path-for ssr-routes/only-routes ::route/toggle-amount-mode)
+                 :hx-target "#account-grid-body"
+                 :hx-swap "outerHTML"
+                 :hx-include "closest form"})
+```
+
+### Grid Body
+
+Wrap the account grid rows in a container with id `account-grid-body`:
+
+```clojure
+[:div#account-grid-body
+ (fc/cursor-map #(transaction-account-row* ...))
+ ...total/balance rows...]
+```
+
+When toggled, only this container re-renders. All other form fields (vendor, memo, approval status) are preserved.
+
+## Data Flow
+
+### Toggle Request (HTMX)
+
+1. User clicks toggle
+2. HTMX serializes entire form via `hx-include "closest form"`
+3. POST to `::route/toggle-amount-mode`
+4. Server:
+   - Merges form params into existing `multi-form-state`
+   - Extracts old mode and new mode
+   - Converts all `:transaction-account/amount` values:
+     - If old="$" new="%": multiply by 100/total
+     - If old="%" new="$": use `percentages->dollars` (see Conversion Logic)
+   - Updates `amount-mode` in state
+   - Re-renders `#account-grid-body`
+5. Client swaps grid body. Tab order preserved.
+
+### Conversion Logic
+
+**$ → %:**
+```clojure
+(defn ->percentage [amount total]
+  (when (and amount total (not= total 0))
+    (* 100.0 (/ amount total))))
+```
+
+**% → $ (using spread-cents):**
+```clojure
+(defn percentages->dollars [percentages total]
+  (let [total-cents (int (* 100 (Math/abs total)))
+        pct-sum (reduce + 0 percentages)
+        ;; Normalize percentages to sum to 100
+        normalized-pcts (if (zero? pct-sum)
+                          (repeat (count percentages) 0)
+                          (map #(* (/ % pct-sum) 100) percentages))
+        ;; Convert each pct to its share of cents
+        individual-cents (map #(int (* total-cents (/ % 100))) normalized-pcts)
+        short-by (- total-cents (reduce + 0 individual-cents))
+        ;; Distribute remainder using spread-cents pattern
+        adjustments (concat (take short-by (repeat 1)) (repeat 0))
+        final-cents (map + individual-cents adjustments)]
+    (map #(* 0.01 %) final-cents)))
+```
+
+Example: One account at 100% of $200.00 → `total-cents=20000`, `individual-cents=[20000]`, result: `[200.00]`
+
+### Save Handling
+
+Before validation in `save-handler :manual`:
+
+```clojure
+(let [snapshot (:snapshot multi-form-state)
+      accounts (:transaction/accounts snapshot)
+      total (Math/abs (:transaction/amount existing-tx))
+      mode (:amount-mode snapshot "$")
+      ;; If in % mode, convert back to $ before saving
+      accounts' (if (= "%" mode)
+                  (let [percentages (map :transaction-account/amount accounts)
+                        dollar-amounts (percentages->dollars percentages total)]
+                    (map #(assoc %1 :transaction-account/amount %2) accounts dollar-amounts))
+                  accounts)]
+  ...)
+```
+
+## Form Preservation
+
+The HTMX toggle is designed to preserve:
+- **Tab order:** All inputs remain in DOM with same `tabindex` attributes
+- **Other form fields:** Vendor, memo, approval status are outside `#account-grid-body`
+- **Alpine.js state:** `x-data` on rows uses `data-key="show"` for animation—this is re-established on re-render
+- **Field names:** Account/location/amount field names follow `step-params[transaction/accounts][N][...]` pattern
+
+## Error Handling
+
+- **Zero transaction amount:** If total is $0, percentages are all 0%. Toggle is disabled or shows error.
+- **Percentage sum ≠ 100:** After editing in % mode, if percentages don't sum to 100, normalize proportionally before converting back to $.
+- **Invalid input:** If user types non-numeric in % mode, existing form validation catches it on submit.
+
+## Testing Strategy
+
+1. **Toggle $→%:** 200/200 transaction shows 100.0
+2. **Toggle %→$:** 100% on 200 transaction shows 200.00
+3. **Multiple accounts:** 50/50 split on 200 → 100.00/100.00 after conversion
+4. **Cent distribution:** 33.33/33.33/33.34% on $100 → uses spread-cents for accurate distribution
+5. **Form preservation:** Toggle doesn't lose vendor/memo data
+6. **Save in % mode:** Correctly converts back to $ before Datomic transaction
+
+## Files to Modify
+
+- `src/clj/auto_ap/ssr/transaction/edit.clj` — main implementation
+- `src/clj/auto_ap/routes/transactions.clj` — add `::route/toggle-amount-mode`
+- `src/clj/auto_ap/ssr/transaction/edit.clj` routes map — register handler
+
+## Future Considerations
+
+- This pattern could be extracted for reuse in invoice expense accounts
+- Consider persisting user's last-used mode preference in localStorage
+- Could add visual indicator when percentages don't sum to 100%