feat(security): harden API export, remove global flags, unify scheduler
- Removed reliance on window.DEBUG_MODE and window.DISABLE_* flags. - Configuration is now passed via constructor instead of global variables. - Exposed API remains on `window.secureBitChat`, but without backup globals or hidden toggles. - Consolidated multiple setInterval tasks into a single scheduler to reduce attack surface. - Added strict limits for IV history and log storage to prevent memory exhaustion. - Improved XSS hardening: no more global switches to disable security features.
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