Fix distill-replace creating new conversation entry in UI

[okottorika]

Problem

The "Distill & Replace in Place" feature was creating a new conversation entry in the sidebar when initiated, which was confusing to users. The UI immediately navigated to the newly created conversation before the slug swap completed, resulting in a blank conversation appearing in the sidebar temporarily.

Solution

This PR fixes the issue by deferring navigation until the distill-replace operation completes:

Frontend Changes (ui/src/App.tsx)

• Track active distill-replace operations: Added state to maintain a mapping of source conversation IDs to their replacement conversation IDs
• Deferred navigation: Instead of immediately navigating to the new conversation, the UI now stays on the source conversation and waits for the SSE conversation list update indicating the source has been archived
• Automatic navigation on completion: When the source conversation is archived (signaling the slug swap is complete), the UI automatically navigates to the new conversation, which now has the original slug

Test Improvements (server/distill_test.go)

• Added proper waiting loops for asynchronous slug swap and archival operations in:
  • TestDistillReplaceConversation
  • TestDistillReplaceConversationNoSlug
  • TestDistillReplaceMultiPass

These tests were checking assertion results before the async operations completed, causing intermittent failures.

User Experience

Before: User clicks "Distill & Replace in Place" → New blank conversation appears in sidebar → Confusing intermediate state → Eventually gets correct conversation

After: User clicks "Distill & Replace in Place" → Stays on current conversation → Slug swap completes in background → Automatically navigates to the distilled conversation with the original slug

Testing

• All existing distill-replace tests pass
• TypeScript type checking passes
• UI builds successfully
• Manually verified the fix with a local Shelley instance

Fixes #158

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