CLAUDE.md: add UI section covering Pivot inspector patterns

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -140,6 +140,28 @@ records.data → apply_transformations() →
 - Server.js has global error handler
 - Database functions return JSON with `success` boolean
 
+## UI (React + Vite)
+
+The frontend lives in `ui/src/` and is built to `public/` via `npm run build` from the `ui/` directory. **Always run `npm run build` from `ui/` after any changes to `ui/src/` files.**
+
+### Pages
+
+- **Sources / Rules / Mappings / Records** — standard CRUD pages
+- **Pivot** (`ui/src/pages/Pivot.jsx`) — interactive pivot/crosstab powered by Perspective (`@perspective-dev` v4.4.0, loaded from CDN). See `docs/perspective-pivot.md` for the full Perspective API reference.
+- **Stacks** — multi-source union views with running balance
+- **Log** — import audit trail
+
+### Pivot inspector panel
+
+Clicking a data cell opens a right-hand inspector panel showing the underlying transactions for that cell. Key behaviors:
+
+- **Toggle**: clicking the same cell again closes the panel. The toggle key is `JSON.stringify({ p: row.__ROW_PATH__, c: column_names })` — stable across source and stack views.
+- **Listener cleanup**: the `perspective-click` handler is stored in `perspClickHandlerRef` and removed via `removeEventListener` on effect cleanup. Without this, switching views accumulates duplicate listeners that fire multiple times per click.
+- **split_by filter derivation**: `detail.config.filter` from the click event may omit split_by column constraints. They are derived from `column_names` positionally (`column_names[i]` matches `config.split_by[i]`) and appended to the filter before querying.
+- **Row filtering**: a temporary `table.view({ filter, expressions })` is used so Perspective evaluates expression/computed columns correctly. Falls back to JS-side `filterRowsByConfig` on error (which skips filters for fields not in raw data).
+- The panel is resizable via a drag handle on its left edge (`paneWidth` state, min 240px).
+- The transaction table is sortable (click header) and shows column totals for all-numeric columns.
+
 ## File Structure
 
 ```
@@ -154,6 +176,14 @@ dataflow/
 │       ├── rules.js
 │       ├── mappings.js
 │       └── records.js
+├── ui/
+│   ├── src/
+│   │   ├── pages/         # One file per page
+│   │   └── api.js         # API client
+│   └── package.json
+├── public/                # Built UI (gitignored, generated by npm run build)
+├── docs/
+│   └── perspective-pivot.md  # Perspective API reference
 ├── examples/
 │   ├── GETTING_STARTED.md # Tutorial
 │   └── bank_transactions.csv