docs: add design spec for unified add/edit stock modal

Job Edit page's Stock tab currently has three inconsistent entry points
(bulk import modal, inline custom-length form, inline edit forms) for
what should be a single add/edit stock flow.

docs/superpowers/specs/2026-08-01-unified-add-stock-modal-design.md

@@ -0,0 +1,74 @@
+# Unified Add/Edit Stock Modal — Job Edit Page, Stock Tab
+
+## Problem
+
+The Stock tab on the Job Edit page (`CutList.Web/Components/Pages/Jobs/Edit.razor`) currently exposes three separate, inconsistent UI surfaces for adding/editing job stock:
+
+1. **Import from Inventory** button → opens a full modal (`showImportModal`) with a bulk multi-select table (grouped by material, checkboxes, Select All/None, per-row qty/priority). Disabled entirely when the job has no parts.
+2. **Add Custom Length** button → toggles an inline (non-modal) single-row form (`showCustomStockForm`) in the card body.
+3. **Edit** (pencil icon on an existing stock row) → reuses one of two inline single-row forms depending on the item type: the bulk-import modal's sibling single form (`showStockForm`, inventory-sourced) or the custom form (`showCustomStockForm`), neither as a modal for the inventory case's add path (only reachable via Edit today).
+
+This is confusing: two different buttons for adding, plus edit behavior that doesn't match either add flow.
+
+## Goal
+
+Consolidate all of this into a single **"Add Stock"** button and one modal that handles both adding and editing, for both inventory-sourced and custom-length stock.
+
+## State Model
+
+Replace the three separate visibility flags with:
+
+- `showStockModal` (bool) — single modal visibility flag, replacing `showStockForm`, `showCustomStockForm`, and `showImportModal`.
+- `stockModalTab` (enum: `Inventory`, `Custom`) — which tab is active. Only switchable in Add mode.
+- `editingStock` (existing field) — `null` means Add mode; non-null means Edit mode, and fixes which tab is shown.
+
+`newStock`, `importCandidates`, `stockErrorMessage`, `importErrorMessage`, `availableStockItems`, `stockSelectedShape`, `stockSelectedMaterialId` are all reused as-is from the current implementation.
+
+## Header
+
+The "Import from Inventory" and "Add Custom Length" buttons are replaced with a single **"Add Stock"** button. It is always enabled (not gated on `job.Parts.Count`). Clicking it:
+
+- Sets `editingStock = null`.
+- Opens the modal (`showStockModal = true`).
+- Defaults `stockModalTab` to `Inventory` if `job.Parts.Count > 0`, otherwise `Custom`.
+
+## Modal — Add Mode (`editingStock == null`)
+
+Nav tabs shown: **From Inventory** | **Custom Length**.
+
+- The **From Inventory** tab is disabled (with the existing tooltip copy, "Add parts first to match against inventory") when `job.Parts.Count == 0`. Its content is today's bulk-select table verbatim, reusing `importCandidates` and the existing `ShowImportModal` loading logic (grouped by material, Select All/None, per-row qty/priority inputs).
+- The **Custom Length** tab content is today's single-row custom form verbatim (shape → size → length → qty → priority).
+- Footer's primary button adapts to the active tab:
+  - Inventory tab: "Import N Item(s)", disabled when no candidates are selected — wired to `ImportSelectedStockAsync`.
+  - Custom tab: "Add Stock" — wired to `SaveCustomStockAsync`.
+- "Cancel" button always present, closes the modal and resets `editingStock`, `newStock`, `importCandidates`, and error messages.
+
+## Modal — Edit Mode (`editingStock != null`)
+
+No tab nav is shown — a single fixed form matching the item's existing type:
+
+- If `editingStock.IsCustomLength`: renders the same custom-form fields (shape → size → length → qty → priority), prefilled from the stock row — wired to `SaveCustomStockAsync`.
+- Else: renders the same single-item inventory form fields (shape → size → stock-length dropdown → qty → priority), prefilled — wired to `SaveStockFromInventoryAsync`.
+- Footer primary button: "Save Changes".
+- "Cancel" button behaves as above.
+
+`EditStock(stock)` sets `editingStock`, prefills `newStock` (as it does today), sets `stockModalTab` to match the item's type, and opens `showStockModal = true`.
+
+## Method Reuse
+
+The four existing handler methods keep their current logic unchanged — only the surrounding modal chrome (open/close state, which form renders) changes:
+
+- `SaveStockFromInventoryAsync` — single inventory-sourced add/edit.
+- `SaveCustomStockAsync` — single custom-length add/edit.
+- `ImportSelectedStockAsync` — bulk inventory import (add only).
+- `ShowImportModal` (the loading logic, not the modal-open flag itself) — populates `importCandidates` when the Inventory tab is active/opened in Add mode.
+
+## Error Handling
+
+Unchanged. Each tab/mode keeps its own existing inline `alert-danger` validation message (`stockErrorMessage` for the two single-item forms, `importErrorMessage` for the bulk table).
+
+## Out of Scope
+
+- No changes to the Parts tab or its modal.
+- No changes to the underlying `JobService` methods or API surface.
+- No change to validation rules or business logic — this is a UI consolidation only.