Disable demo auth when SSO providers are configured

[initstring]

Motivation

• Prevent the insecure demo login from remaining enabled when any SSO provider is configured, reducing risk of accidental public demo access.
• Align provider registration to the validated env surface so server-side feature gating uses the same canonical env values.

Description

• Compute ssoProvidersEnabled and make demoModeEnabled require no SSO in src/server/auth/config.ts, and register providers using env.* instead of process.env.*.
• Update the sign-in page src/app/(public-routes)/auth/signin/page.tsx to hide the demo button when any SSO provider is enabled by computing ssoEnabled and gating demoEnabled accordingly.
• Add a short note to docs/installation.md explaining that demo mode is automatically disabled when any SSO provider is configured.

Testing

• No automated tests were run as part of this change.

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Codex Task

[initstring]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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