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/templates/monolith/monolith.test.tsx

@@ -193,7 +193,7 @@ describe("table, totals and footer", () => {
       expect(amount.text, `row ${i}`).toMatch(/^₹[\d,]+\.\d\d$/);
       expect(amount.right, `row ${i} amount edge`).toBeCloseTo(right - V.table.numPad, 0);
     }
-    const qty = textLines(byId(r, `${TABLE_ROW_PREFIX}0`)).find((l) => l.text === "1")!;
+    const qty = textLines(byId(r, `${TABLE_ROW_PREFIX}0`)).find((l) => l.text === "1 hr")!;
     const plan = planMonolith(r.fixture.model, frameForPrefs(r.prefs), V, null, false);
     const left = frameForPrefs(r.prefs).pad.left + plan.W - plan.cw.amount - plan.cw.qty;
     expect(qty.x + (qty.right - qty.x) / 2).toBeCloseTo(left + plan.cw.qty / 2, 0);