Add the A4 report designer and fix band layout

Brings in the A4 (millimetre) report designer alongside the existing
matrix editor, migrates the frontend from react-scripts to Vite/Vitest,
and fixes four defects found while using the A4 band tools.

Layout — layout.js now implements the standard banded-report model:
everything prints where it was drawn, and a band growing per data row is
the only thing that moves anything. Previously `paginate` re-flowed every
band from the top margin, so a band placed halfway down the page
previewed at the top, and `header` was treated as a page header and
hoisted above the statics drawn before it. A header is a COLUMN header:
it prints in place and repeats only on a continuation page. Statics now
take part too — one drawn below a band keeps its drawn gap as that band
grows, and follows onto the next page rather than vanishing when pushed
past the bottom margin. The displacement is keyed on the band's design
bottom, not on an instance's laid-out y: every row of a band shares one
design position, so keying on the instance measured the gap from row 1
and let the static overlap rows 2..n.

Band children — a child's x is relative to its band, as its y already
was. layout() emitted it raw, so a field printed measured from the paper
edge instead of the band's left margin. Instances now carry bandX.

Band placement — a band's x comes from the page margins, so creation
takes only the click's Y (the factory's x was being overwritten by the
spread order) and dragging is locked to the vertical axis. Bands render
as a strip, with top/bottom rules only, matching the matrix editor.
fromApiReport reflows on load so reports saved with a stray x straighten
themselves. Matrix bands are constrained to vertical too.

Drag lifecycle — useElementDrag re-subscribed its document listeners on
every render during a drag, because the effect depended on an unmemoized
onDrag. Each re-subscribe removed the mouseup listener before adding the
next, measured as add/remove/add/remove/add across two mousemoves, and a
release landing in that gap was lost: the element kept following the
cursor until the next click. Snapping made it routine, since a held
guide stops x/y changing and leaves the guide overlay's render sitting
between the move and the release. onDrag and dragData now go through
refs so the effect depends on [isDragging] alone.

Note: the matrix editor has no test coverage, so its band constraint is
verified by reading only. The A4 suite is 201 passing tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.claude/agent-memory/react-expert-dev/vite-vitest-migration.md

@@ -0,0 +1,52 @@
+---
+name: vite-vitest-migration
+description: Frontend build is Vite + Vitest (not CRA/Jest) — the JSX-in-.js trap, why axios no longer needs stubbing, and the jsdom border assertion gotcha
+metadata:
+  type: project
+---
+
+The frontend migrated from Create React App / Jest to **Vite + Vitest**. Config
+lives in `frontend/vite.config.mjs` (`.mjs`, not `.js` — Vite's native config
+loader warns about ESM in a `.js` config).
+
+Replaces the old `jest-axios-esm` note, which is now obsolete.
+
+**Vitest transforms axios fine, so the old blanket `jest.mock('axios')` hack is
+gone.** Three of the six a4 suites dropped it entirely. `a4EntryPoints` still
+mocks axios, but for a *behavioural* reason: it drives the report auto-load via
+`api.get.mockResolvedValue(...)`, so the instance `services/api.js` builds at
+import time must be a mock. Distinguish the two reasons before deleting a mock.
+
+**The JSX-in-`.js` trap — this one fails at BUILD time only.** CRA's Babel
+parsed JSX inside `.js` files; Vite does not by default. Setting
+`oxc: { include: /\.(m?ts|[jt]sx|js)$/, lang: 'jsx' }` fixes the dev server and
+Vitest, but **not** `vite build`: the bundled path (Rolldown,
+`builtin:vite-transform`) derives the language from the file extension and
+ignores `lang`. So a `.js` file containing JSX will pass tests and the dev
+server, then fail the production build.
+
+**How to apply:** any new production file containing JSX must be named `.jsx`.
+The `oxc` override exists only so the existing a4 `__tests__/*.test.js` files
+keep working. Importers use extensionless specifiers, so renaming a file
+`.js` → `.jsx` needs no import changes (this is how `print/printReport.jsx` was
+renamed without touching the dynamic `import()` in `Main.jsx`).
+
+**Env vars are `import.meta.env.VITE_*`**, not `process.env.REACT_APP_*`;
+`process.env` does not exist in the browser. The `|| 'http://localhost:…'`
+fallbacks are the offline-deployment safety net — keep them. The dev-server
+port (5172, in `vite.config.mjs`) must match Django's `CORS_ALLOWED_ORIGINS`.
+Renaming these vars means touching `Dockerfile`, `docker-compose.yml`,
+`.env.docker` and the gitignored root `.env` too, or the deployed bundle
+silently falls back to localhost.
+
+**Newer jsdom round-trips `border: none`.** It now reports
+`borderStyle === 'none'` with `borderWidth === 'medium'` (the CSS initial
+keyword). Assert on **`borderStyle`**, not `borderWidth === ''` — the latter was
+written around an old-jsdom quirk and passed vacuously. Two `a4Elements` tests
+had to be corrected for this during the migration; the components were correct.
+
+**`global.IS_REACT_ACT_ENVIRONMENT = true` is no longer required** (Vitest's
+jsdom environment sets it), but it is harmless and still present in all six
+suites.
+
+Related: [[testing-react]], [[a4-print-path]], [[a4-entry-points]]