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

docs/voiced-invoice-v1.md

@@ -33,6 +33,10 @@ Notes on the fields:
 
 - **Items.** `taxable_value` of a line is the line amount before the invoice-level discount. `totals.taxable_value` is
   `subtotal - discount`, the base the tax is charged on. A fixed-amount line has `quantity "1"` and `unit null`.
+- **Units.** On a rate line `unit` is a built-in id (`unit`, `piece`, `set`, `session`, `second`, `minute`, `hour`, `day`,
+  `week`, `month`, `km`, `m`, `sqft`, `sqm`, `kg`, `litre`) or the custom text typed on the invoice (at most 12 characters:
+  letters, digits, spaces and `. / -`). `quantity` may be fractional; a time typed as `4:20` hours is stored as the exact
+  repeating decimal.
 - **UTGST.** When the supplier is in a union territory without a legislature (state codes 04, 26, 31, 35, 38) the second
   head is reported under `utgst` and `sgst` is zero.
 - **Client details.** The name, GSTIN, printed address and place of supply are frozen on the invoice. The structured