#5097: fix(ui): prefer session label over displayName in dropdown

## Summary When using sub-agents with labels (via `sessions_spawn`), the Gateway Dashboard session dropdown displays the raw session key (e.g., `webchat:g-agent-main-subagent-b0b644c5-...`) rather than the human-readable `label` field. ## Changes - Modified `ui/src/ui/views/sessions.ts` to prioritize `label` field over `displayName` - Fallback chain: `label` → `displayName` → `key` ## Before Session dropdown showed: - `webchat:g-agent-main-subagent-b0b644c5-...` - `webchat:g-agent-main-subagent-a1d2e3f4-...` ## After Session dropdown now shows: - `dept-engineering` ✅ - `dept-finance` ✅ ## Testing - [x] `pnpm lint` passes with no errors - [ ] Manual testing (requires Gateway setup) ## Use Case Running a department system with multiple labeled sub-agents becomes much easier when you can identify sessions by their labels rather than UUID-based keys. Fixes #5088 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the Gateway Dashboard sessions table to display a human-friendly session label in the “Key” column when available. Specifically, `renderRow` now chooses the session name using `label → displayName → key`, so labeled sub-agent sessions don’t appear as long raw session keys in the dropdown/table, while links and routing still use the underlying `row.key` for URL params. The change is localized to `ui/src/ui/views/sessions.ts` and does not modify session identity, navigation, or patch/delete behavior—only the displayed text for the session link. <h3>Confidence Score: 4.5/5</h3> - This PR is safe to merge with minimal risk. - The change is a small, localized UI-only tweak that adjusts a display fallback chain without altering session identifiers, routing, or mutation logic. The only noteworthy behavioral difference is that `label` values that are empty/whitespace will now be treated as truthy and displayed, potentially resulting in blank-looking entries, but this is limited to presentation. - ui/src/ui/views/sessions.ts <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->