feat(memory): require review for memory proposals

[delkc]

Summary

Require an explicit review decision before an agent suggestion becomes durable or recallable memory.

• Keep agent suggestions in a local pending queue until the person approves them
• Provide editable approval and decline controls in Settings → Memory
• Show Review and Decline notifications, including session-safe handling for MCP proposals
• Use salted, content-free suppression fingerprints for declines and unambiguous manual deletions
• Serialize and idempotently apply approval in Rust, with credential validation and scoped destination routing

Related issue

N/A. Depends on #288.

Testing

• Proposal parsing, review, suppression, and notification tests
• Backend approval retry, credential rejection, and queue tests
• TypeScript typecheck, i18n, Biome, and design-system checks

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