Choose a file format and background when creating a document

[dariomory]

Fixes #1

The New dialog offered width and height and nothing else, and every document saved as PNG regardless of what you wanted.

What changed

Documents now carry a format — PNG, JPEG or WebP — chosen when the document is created, and Save As proposes the matching extension. The background is selectable too: Transparent, White or Black.

Picking JPEG disables Transparent and says why, since JPEG has no alpha channel.

New
  Width       1920
  Height      1080
  Format      [PNG  v]
  Background  [White v]

Two supporting changes

Transparency now survives export. flattenDocumentToCanvas unconditionally painted the background colour, so a transparent document could never produce a transparent PNG. It now skips the fill when the background alpha is zero, and composites onto white only when the target format cannot carry alpha.

A deliberately created document no longer gets the sample overlay. That decorative golden circle belongs to the document seeded at startup. On a transparent canvas it reads as debris, and on any new document it is something to delete before starting. seedBlankDocument keeps it for the startup document only.

Verification

Driven in a browser: the dialog exposes both selects; selecting JPEG marks the Transparent option disabled; creating with Transparent produces a single Background layer rather than two.

initial:     format=png   background opts: transparent, white, black
after jpeg:  format=jpeg  background opts: transparent(disabled), white, black
layers after create: 1

pnpm test (21) and pnpm exec tsc --noEmit are clean.

Branched off master and independent of #13 and #14, so all three can merge in any order.