design: Scriptty-branded loading spinner + PDF-export progress indicator

[stultus]

What's there now: Long operations (PDF export, file open of a large series) currently rely on the browser's default cursor or a generic disabled state — ExportModal.svelte shows a .spinner-on-primary element with no bespoke art, and there's no global loader pattern.

Why it matters: PDF export of a long screenplay can take 1–3 seconds (Typst compile). That's the only "wait" surface in the app — and it's currently the most generic. A bespoke spinner is a 1-off design moment that punches above its weight.

Suggested deliverable:

1. Spinner mark — small (16–20px) loading indicator in the brand mark style. Could be the ഋ rotating, a stylized film-reel turning, or a typewriter carriage moving.
2. (Optional) progress wrapper — animated wrapper around primary buttons during async work (Save, Export). A subtle progress sweep across the button surface.
3. (Optional) update-toast micro-animation — UpdateToast.svelte currently slides in plain. A 1–2 frame motion accent (the new-version glyph appearing) would make it feel less like an OS banner.

Code touchpoints: src/lib/components/{ExportModal,UpdateToast}.svelte; future src/lib/components/Spinner.svelte.