refactor(frontend): replace the isRepoCard boolean with a card category

[marcalexiei]

Two places independently hardcoded the same "is this a repo or gist card?" check:

• Home.tsx: which theme variant to default to
• Customize.tsx: whether to offer "Show Owner?"

Both now derive it from one map in models/CardType.ts:

export const CardCategory = { REPO: "repo", USER: "user" } as const;

export const CATEGORY_BY_CARD_TYPE: Record<CardType, CardCategory> = { ... };

A new card type won't compile until it's categorised, instead of silently falling into the non-repo branch like the boolean did.
It also leaves room for another category, if we ever need one that changes the data a card requires.

Also:

• ThemeStage takes category instead of isRepoCard.
• getDefaultTheme / onThemeChange use core's ThemeName instead of string.

[vercel]

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