Preserve thread visited state across partial syncs

[hogeheer499-commits]

What changed

• Preserve threadLastVisitedAtById entries when syncThreads receives a partial snapshot that does not include a thread.
• Keep the existing pruning behavior for threadChangedFilesExpandedById.
• Add a regression test for the partial-snapshot visited-state case.

Why this should exist

This is a small, focused frontend state bug fix.

A production crash report showed React error 185 with markThreadVisited in the stack. On current main, ChatView calls markThreadVisited(threadKey, completedAt), but syncThreads could delete that completed-turn timestamp when a partial/recovery snapshot omitted the active thread. After that, the same completed turn could be marked visited again instead of remaining idempotent.

The regression test proves the exact state transition:

1. mark a thread visited at completedAt
2. run syncThreads([]) as a partial snapshot
3. call markThreadVisited(thread, completedAt) again

With the old pruning behavior, the timestamp is lost. With this change, it is preserved and the second mark returns the same state object.

Scope

• No UI or layout change.
• Does not change WebSocket reconnect behavior.
• Does not change server/provider turn lifecycle behavior.
• Separate from the server projection issue tracked in later work.

Validation

• npx --yes bun@1.3.11 run --filter @t3tools/web test src/uiStateStore.test.ts
• npx --yes bun@1.3.11 fmt
• npx --yes bun@1.3.11 lint - exit 0, existing warnings only
• npx --yes bun@1.3.11 typecheck

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[macroscopeapp]

Approvability

Verdict: Approved

Straightforward bug fix that preserves thread visited state across partial syncs by removing incorrect pruning logic. The change is small, localized, and includes updated tests verifying the expected behavior.

^{You can customize Macroscope's approvability policy. Learn more.}

Dismissing prior approval to re-evaluate 60a6d68

[hogeheer499-commits]

Adding the original reporter error details here as reviewer context. The screenshot showed the T3 Code Alpha error boundary with this expanded stack:

Hide error details
Error: Minified React error #185; visit https://react.dev/errors/185 for the full message or use the non-minified dev environment for full errors and additional helpful warnings.
    at Zr (http://192.168.2.13:3773/assets/index-CVbnJsIO.js:9:27497)
    at Yr (http://192.168.2.13:3773/assets/index-CVbnJsIO.js:9:27070)
    at zo (http://192.168.2.13:3773/assets/index-CVbnJsIO.js:9:52284)
    at http://192.168.2.13:3773/assets/index-CVbnJsIO.js:9:52165
    at http://192.168.2.13:3773/assets/index-CVbnJsIO.js:25:79722
    at Set.forEach (<anonymous>)
    at r (http://192.168.2.13:3773/assets/index-CVbnJsIO.js:25:79711)
    at markThreadVisited (http://192.168.2.13:3773/assets/index-CVbnJsIO.js:45:85530)
    at http://192.168.2.13:3773/assets/index-CVbnJsIO.js:306:6630
    at Lc (http://192.168.2.13:3773/assets/index-CVbnJsIO.js:9:91969)

This is the reason I focused this PR on the markThreadVisited path rather than on reconnect handling in general.