Add units of measure and fractional quantities to rate lines

Quantities can now be time, count, distance, area, weight or volume, or a custom unit, instead of only
second/minute/hour/session/unit.

- Registry in src/lib/units.ts (mirrored in src-tauri/src/units.rs): grouped built-in units plus a custom unit of 1-12
  characters. A rate line's unit is validated when an invoice or a preset is saved; migration M12 drops the old unit
  allow-list from item_presets and keeps every row.
- Time is typed as h:mm: "4:30" hours counts as 4.5, "4:20" as 4.333... (kept exact, so rate x quantity rounds once, the
  same in TypeScript and Rust). Other units take plain decimals up to 3 places. A bad quantity blocks issuing.
- The PDF quantity column shows the unit ("4.5 hr", "12.75 km"); the rate keeps "per hour" / "/hr". RenderItem carries
  priceText, perText and rateShort, so no template parses the rate text with a regex over five hard-coded units.
- Quantity columns are sized from the widest word once a quantity with its unit is wider than 72 pt, so one long
  custom unit cannot squeeze the description (the serenity and citrus-split tables overflowed in the new fixture).
  Classic's quantity column is now content-sized.
- ERPNext: the default UOM map comes from the registry, with fractional-capable UOMs for time, distance, area, weight
  and volume. A custom unit is sent as Nos. A quantity that is not exact to 3 decimals (4:20 hours) is still refused for
  push, as before.
- Goldens and template thumbnails regenerated; the template harness has a "units" fixture with the longest texts.

Claude-Session: https://claude.ai/code/session_01PZypiWDfMkDTeEPeXjRhW5

src/views/NewInvoice.tsx

@@ -34,7 +34,8 @@ import {
 } from "@carbon/icons-react";
 import { confirm } from "@tauri-apps/plugin-dialog";
 import { api } from "../lib/api";
-import { EMPTY_ITEM, INDIAN_STATES, UNIT_LABELS } from "../lib/types";
+import { EMPTY_ITEM, INDIAN_STATES } from "../lib/types";
+import { isValidUnit } from "../lib/units";
 import type {
   BankAccount,
   Client,
@@ -44,7 +45,6 @@ import type {
   InvoiceInput,
   InvoiceItem,
   LineMode,
-  LineUnit,
   Settings,
   TaxType,
 } from "../lib/types";
@@ -57,6 +57,8 @@ import { formToPreviewInvoice } from "../lib/previewInvoice";
 import { fromDraftPayload, toDraftPayload, type InvoiceFormState } from "../lib/draft";
 import { SHORTCUT_LIST, useShortcuts } from "../hooks/useShortcuts";
 import PageSetupControls from "../components/PageSetupControls";
+import QuantityInput from "../components/QuantityInput";
+import UnitPicker from "../components/UnitPicker";
 import { useInvoicePdf } from "../hooks/useInvoicePdf";
 import { useInvoiceExport } from "../hooks/useInvoiceExport";
 import { autoPushIssued } from "../lib/erpnextUi";
@@ -257,6 +259,12 @@ export default function NewInvoice({ settings, onSettingsChange, active, onActiv
       ? "cgst_sgst"
       : "igst";
   const clientGstinError = gstinFullError(clientGstin);
+  // Quantity boxes that hold text which is not a quantity; they keep the last good value, so issuing must wait.
+  const badQuantities = useRef(new Set<string>());
+  const markQuantity = (id: string, ok: boolean) => {
+    if (ok) badQuantities.current.delete(id);
+    else badQuantities.current.add(id);
+  };
   const totals = useMemo(
     () => computeTotals(items, discount, effectiveTaxType, taxRate),
     [items, discount, effectiveTaxType, taxRate],
@@ -614,6 +622,14 @@ export default function NewInvoice({ settings, onSettingsChange, active, onActiv
       setError(`Client GSTIN: ${clientGstinError}`);
       return;
     }
+    if (badQuantities.current.size > 0) {
+      setError("A quantity is not a number. Fix the line items marked in red.");
+      return;
+    }
+    if (items.some((i) => i.mode === "rate" && !isValidUnit(i.unit))) {
+      setError("A line item has no valid unit. Pick one, or type a custom unit.");
+      return;
+    }
     setSaving(true);
     setError(null);
     exp.dismiss();
@@ -997,28 +1013,22 @@ export default function NewInvoice({ settings, onSettingsChange, active, onActiv
                         onChange={(_e, { value }) => updateItem(index, { rate: Number(value) || 0 })}
                       />
                     </div>
-                    <div style={{ flex: "0 1 140px" }}>
-                      <Select
+                    <div style={{ flex: "0 1 160px" }}>
+                      <UnitPicker
                         id={`unit-${index}`}
                         disabled={locked}
-                        labelText="Unit"
                         value={item.unit}
-                        onChange={(e) => updateItem(index, { unit: e.target.value as LineUnit })}
-                      >
-                        {Object.entries(UNIT_LABELS).map(([value, text]) => (
-                          <SelectItem key={value} value={value} text={text} />
-                        ))}
-                      </Select>
+                        onChange={(unit) => updateItem(index, { unit })}
+                      />
                     </div>
-                    <div style={{ flex: "0 1 110px" }}>
-                      <NumberInput
+                    <div style={{ flex: "0 1 130px" }}>
+                      <QuantityInput
                         id={`qty-${index}`}
                         disabled={locked}
-                        label="Quantity"
-                        min={0}
-                        step={1}
                         value={item.quantity}
-                        onChange={(_e, { value }) => updateItem(index, { quantity: Number(value) || 0 })}
+                        unit={item.unit}
+                        onChange={(quantity) => updateItem(index, { quantity })}
+                        onValidity={(ok) => markQuantity(`qty-${index}`, ok)}
                       />
                     </div>
                   </>