[Feature] Allow customizing the OIDC login button text #5270

Is your feature request related to a problem? Please describe.
When using a generic OIDC provider (OIDC_PROVIDER=generic), the login button text is hardcoded to "Login with Generic" — it's derived automatically by capitalizing the OIDC_PROVIDER value, which is itself a fixed enum (authelia/authentik/keycloak/pocket-id/zitadel/voidauth/generic), not freeform text. There's no way to set it to anything else.

This looks odd for self-hosted setups using a custom/in-house OIDC bridge instead of one of the named providers above — "Login with Generic" doesn't tell users anything about who's actually issuing the login (in my case, a custom bridge that authenticates via Plex). Every other self-hosted app I've wired up to the same bridge (Audiobookshelf, Calibre-Web-Automated, Shelfmark) lets me set a custom button label; RomM is the only one that doesn't expose this.

Describe the solution you'd like
Add an OIDC_BUTTON_TEXT (or similarly named) environment variable that, when set, overrides the default "Login with {OIDC_PROVIDER}" text on the login button. Fall back to the current capitalized-provider-name behavior when it's unset, so this is fully backwards compatible.

Describe alternatives you've considered

• Patching the compiled frontend directly to change the hardcoded label — works, but has to be redone on every image update, so it's not a real long-term option.
• Picking a different OIDC_PROVIDER value instead of generic — not viable, since it's a fixed enum and none of the other options describe a custom/in-house provider.

Additional context
None of the existing closed OIDC issues (#1941, #3564, #2346, etc.) cover button label customization specifically — they're about provider support, RBAC, and bug fixes rather than this.

^^this been drafted by claude. I am hoping the technical bits are more or less correct, as i have not dug into the weeds more than realizing there was no way to change the text without a fork. thanks!