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/pdf/model/build.test.ts

@@ -133,7 +133,10 @@ describe("unregistered invoice", () => {
       descriptionLines: ["Voice-over", "session A"],
       hsnSac: "998397",
       rateText: "₹1,200.00 per hour",
-      qtyText: "2.5",
+      priceText: "₹1,200.00",
+      perText: "per hour",
+      rateShort: "₹1,200.00/hr",
+      qtyText: "2.5 hr",
       amountText: "₹3,000.00",
     });
     expect(m.items[1]).toEqual({
@@ -141,10 +144,22 @@ describe("unregistered invoice", () => {
       descriptionLines: ["—"],
       hsnSac: null,
       rateText: null,
+      priceText: null,
+      perText: null,
+      rateShort: null,
       qtyText: "1",
       amountText: "₹2,500.50",
     });
   });
+  it("words the unit and the quantity for every kind of unit", () => {
+    const line = (unit: string, quantity: number, rate = 18.5) =>
+      make({ items: [{ ...base.items[0], mode: "rate", unit, quantity, rate }] }).items[0];
+    expect(line("km", 12.75)).toMatchObject({ qtyText: "12.75 km", rateText: "₹18.50 per km", rateShort: "₹18.50/km", perText: "per km" });
+    expect(line("sqft", 1250.5)).toMatchObject({ qtyText: "1250.5 sq ft", rateShort: "₹18.50/sq ft" });
+    expect(line("hour", 4 + 1 / 3)).toMatchObject({ qtyText: "4:20 hr" });
+    expect(line("unit", 3)).toMatchObject({ qtyText: "3", rateText: "₹18.50 per unit", rateShort: "₹18.50/unit" });
+    expect(line("  bundle ", 2)).toMatchObject({ qtyText: "2 bundle", rateText: "₹18.50 per bundle", rateShort: "₹18.50/bundle" });
+  });
   it("labels the bank lines and defaults the signature to the vendor name", () => {
     expect(m.bank).toEqual({
       accountName: "Test Vendor",