agama-project / agama / 30261829568

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fix(web): avoid a flash from Agama colors to a product's own palette (#3749)

## Problem

When a product ships a custom appearance skin
(`assets/products/<id>.css`, see `doc/product_theming.md`), the browser
paints with Agama's stock colors first and only switches to the
product's palette once the product config
resolves from the backend and `useProductAppearance` injects the
stylesheet link. Anything rendered before that (most visibly the
full-screen loading spinner) shows the wrong colors, then flips. This is
a visible flash on
every load/reload of a themed product.

Light/dark/high-contrast already avoid the equivalent problem: a small
inline script in `index.html` applies the persisted choice before React
boots. The product skin had no equivalent.

## Fix

- `use-product-appearance.ts` now mirrors the active product id to
`localStorage` (`agm-product-id`), alongside the existing behavior of
injecting/updating the `<link rel="stylesheet"
href="assets/appearance/<id>.css">`.
- `index.html` gains a second pre-boot script, matching the existing
light/dark one: it reads `agm-product-id` and, if present, creates that
same `<link>` element (same id) before first paint. Once the app boots
and the real product resolves, `useProductAppearance` finds the existing
element by id and updates it in place rather than creating a duplicate.
- A missing/404 stylesheet is already harmless (documented in
`doc/product_theming.md`), so an unthemed product or a first-ever visit
behaves exactly as before.

Known, accepted trade-off: if the product changes between two loads, the
pre-paint script briefly shows the *previous* product's colors until the
real query resolves and swaps the stylesheet. Same category of trade-off
the existing script already accepts for OS-level "system" preference
changes between loads.

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