feat: secure logger and group list sidebar active state

[patopatrish]

Summary

• Implements a centralized, privacy-safe logging service (lib/logger.ts) with automatic redaction of sensitive fields
• Enhances the group list sidebar with a visually distinct active-group state, group avatars, and improved last-message layout

Changes

lib/logger.ts — new file

• Configurable log level via LOG_LEVEL env var (debug | info | warn | error; defaults to info)
• Structured JSON log entries: { timestamp, level, event, correlationId?, context? }
• Deep sanitizer that redacts any field whose key matches a sensitive set — ip, content, message, token, email, metadata, password, authorization, etc. — at any nesting depth, so no PII or message content ever reaches the log output
• Four-method surface: logger.debug / .info / .warn / .error

app/chat/page.tsx — sidebar enhancement

• Each group row now shows a circular avatar with the group's first letter (filled with primary colour when active, muted otherwise)
• Active group gets an inset left-border accent (shadow-[inset_3px_0_0_hsl(var(--primary))]) + bolder name, making the selected room unmistakably distinct
• Last-message preview and timestamp/unread badge rearranged for a cleaner two-row layout

Closes

Closes #94
Closes #105

Test plan

• Visit /chat — verify groups sidebar renders with avatars and last-message previews
• Click a group — confirm the active-state highlight (left accent + avatar fill + bold name) appears and switches correctly
• Import logger in any server file and call logger.info("test", { ip: "1.2.3.4", content: "hello" }) — confirm both fields are [REDACTED] in output
• Set LOG_LEVEL=warn and confirm logger.info(...) calls produce no output
• Set LOG_LEVEL=debug and confirm all levels emit

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