fix(desktop): keep completed todo panel visible

[GTC2080]

Fixes #2942

Summary

• Keep the desktop todo panel visible for the latest successful todo_write, including the final all-completed update.
• Preserve user dismissal behavior by still keying the hidden state to the todo item id.
• Add a focused visibility regression check with a small performance loop.

Verification

• npm run test:todo-visibility (200,000 visibility checks in 1.08 ms, 0.005 us/call)
• npm run typecheck
• npm run build (Vite built in 7.09s; wrapper elapsed 11,685 ms)
• D:\Go\go\bin\go.exe test ./internal/agent ./internal/evidence ./internal/tool/builtin -count=1
• D:\Go\go\bin\go.exe vet ./...
• D:\Go\go\bin\go.exe test ./... -p 1 -count=1
• git diff --check

Note: a plain concurrent go test ./... -count=1 on this Windows host repeatedly timed out the unrelated internal/cli custom statusline test under package-level load; internal/cli passed standalone, and the full suite passed with -p 1.