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/a4-designer-conventions.md

@@ -0,0 +1,44 @@
+---
+name: a4-designer-conventions
+description: Non-obvious invariants of the A4 report designer — line element shape, mm-vs-px flooring, and why matrix inspector panels must not be imported
+metadata:
+  type: project
+---
+
+Conventions in `frontend/src/components/ReportEditor/a4/` that are easy to get
+wrong because they contradict the matrix editor sitting next to them.
+
+**Lines carry only one dimension.** An `a4-hline` has `width` + `thickness`
+and NO `height`; an `a4-vline` has `height` + `thickness` and NO `width`.
+Anything computing a box from an element must substitute `thickness` for the
+missing axis — `getA4Bounds` in `geometry.js` does this, and it is the
+function to reach for rather than reading `el.width`/`el.height` directly.
+Reading the raw props for a line compares against `undefined` and passes
+vacuously in tests while being wrong in the product.
+
+**Screen floors hairlines, print must never.** Screen renderers clamp a line's
+painted thickness and a rect's border to `Math.max(1, …)` px, because 0.3mm at
+50% zoom is 0.57px and browsers can round it away to nothing. `PrintDocument`
+deliberately does no such thing — the printed thickness is what a steel rule
+measures. If a test asserts a floored value on the print path, the test is
+wrong, not the code.
+
+**Do not import the matrix inspector panels.** `ExpressionEditorPanel.jsx`
+looks tempting for the A4 expression section but is coupled two ways: it finds
+a parent band with `el.type === 'band'` (A4 bands are `a4-band`) and its
+alignment select writes `element.alignment`, whereas every A4 element and
+renderer reads `element.align`. Reusing it silently creates a dead second
+alignment property. The duplication between the two inspectors is accepted
+deliberately — see the plan's "matrix editor is not modified" section.
+
+**`fillColor: null` means no fill,** distinct from white, which would hide
+whatever a rect is framing. A colour input cannot express null, so a checkbox
+owns that state. It round-trips through Django's JSONField as SQL null.
+
+**Float inputs use `MmInput`,** never `<input type="number">`: a number input
+returns `""` for the intermediate value `"12."`, so the decimal point appears
+not to type. `MmInput` (in `A4Inspector.jsx`) holds a draft string while
+focused. Font size in pt is the one exception — its step is 0.5 and users
+never type a long decimal tail there.
+
+Related: [[vite-vitest-migration]]