feat(flow-chat): add recursive agent session tree

[wsp1911]

Summary

Add a recursive Agents session tree for FlowChat and replace the legacy background subagent activity panel with a background command panel.

Fixes #

Type and Areas

Type:

Feature / bug fix / UI/UX / test

Areas:

Rust core, desktop/Tauri, server/relay, web UI, localization

Motivation / Impact

Agents can now be viewed as a recursive parent-child session hierarchy.

This PR:

• Adds persisted session lineage APIs with remote workspace support.
• Adds an Agents tree popover with live lifecycle status and session titles.
• Supports per-session cancellation without automatically cancelling descendants.
• Highlights the Agents entry for active foreground and background descendants.
• Preserves manual collapse state while sessions are running.
• Replaces the background activity panel with a background command panel.
• Updates localized labels, icons, styles, and focused tests.

Verification

• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/FlowChatHeader.test.tsx src/flow_chat/components/modern/SessionTreePopover.test.tsx src/flow_chat/utils/sessionLineage.test.ts src/web-ui/src/infrastructure/api/service-api/AgentAPI.test.ts
• pnpm run i18n:audit
• pnpm --dir src/web-ui run lint
• pnpm run theme:color-audit:all
• cargo check -p bitfun-desktop
• cargo check --workspace
• pnpm run check:core-boundaries
• git diff --check

Manual UI interaction verification was not performed per repository instructions.

Reviewer Notes

• cancelDescendants defaults to true for backward compatibility.
• Agents tree cancellation explicitly passes cancelDescendants: false.
• Persisted metadata remains authoritative for display titles while live sessions provide runtime state.
• No database migration is required.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.