feat(ui): configurable default theme

[pasteley]

Add support for setting a default theme (light, dark, auto) via the argocd-cm ConfigMap using the ui.defaulttheme key. The backend setting is exposed through the /api/v1/settings API and consumed by the UI on initialization, allowing operators to configure the default theme without requiring users to manually select it.

Closes: #18524

Checklist:

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[pasteley]

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[nitishfy]

I’m not convinced this should live in the argocd-cm. The ConfigMap is primarily meant for instance-wide, admin-controlled configuration that affects how Argo CD behaves (auth, RBAC, repositories, feature flags, etc.). UI theme preference is fundamentally a user-facing, per-user concern, not a cluster- or instance-level one.

[pasteley]

@nitishfy I can see why a “theme” setting might sound like a per-user preference at first glance, but in this PR it’s explicitly an operator-defined default that’s only used when no user preference exists. If a user selects a theme, that continues to take precedence.

Also, argocd-cm already carries other instance-level UI configuration (custom CSS + banners), e.g.:
https://github.com/argoproj/argo-cd/blob/master/docs/operator-manual/argocd-cm.yaml#L311

So placing the default theme alongside existing UI customization knobs is consistent with current patterns.

If you still think this shouldn’t live in argocd-cm, could you suggest a concrete alternative config surface I should use instead?

[pasteley]

Related: #25985