CPLAT-9497: fix session list single-click selection

[gavin-jeong]

JIRA: https://sendbird.atlassian.net/browse/CPLAT-9497

Summary

• fix session list mouse selection so single clicks select visible items even when filtering is active
• remove the filtered-list click guard that prevented selection in the sessions view
• add a regression test for filtered single-click selection in the TUI

Test plan

• go test ./internal/tui -run TestSessionSingleClickSelectsVisibleFilteredItem -v
• go test ./internal/tui
• manually verify session list single-click selection in the TUI

[jinsekim]

LGTM!