[014] Add network error logging

[konard]

Parent epic: #1

Phase: Connectivity Layer
Priority: 14

Scope

Add structured logging for network and proxy errors while redacting credentials and user content.

Implementation Steps

• Define network error categories and log fields.
• Redact proxy secrets, phone numbers, tokens, and message content.
• Add tests for redaction and error classification.

Acceptance Criteria

• Connectivity failures produce actionable structured logs.
• Secret-like values are redacted from logs.
• Tests cover representative direct, MTProto, and SOCKS5 failures.

Automation

Generated from #1 using scripts/decompose-epic.mjs.