feat: keep delegated sessions accessible from the conversation header

[ker2xu]

Delegated sessions become hard to find once the parent continues chatting: the inline card is folded into earlier work, and the existing sub-agent overlay only follows the latest reply. Add a persistent parent-header action that lists all saved direct child conversations and opens the existing session viewer without scrolling back through the transcript.

The list keeps live child updates, prioritizes blocked/running sessions, recovers from reload/reconnect, and protects against stale history requests, deleted children and status-only events arriving during loading. The sidebar keeps its child count and expand action visible at a stable right-edge position, including when hover quick actions appear. Includes all ten UI locales and regression coverage.

Validation:

• Latest revision 5a08b36e: all seven GitHub CI jobs passed. This covers frontend lint, browser-agent bundle/type checks, the full Vitest suite and standard static export build, plus Rust desktop/server tests and Clippy on Linux, macOS and Windows.
• Added 11 regression tests for historical recovery, live updates/loading races, reconnect, deletion, parent isolation, permissions, retry and viewer focus. Related regression coverage passed; all 33 sidebar-card tests passed after the final pointer-stability adjustment.
• Playwright verified reload recovery, the complete three-child menu, every child viewer, focus restoration, sidebar expansion/collapse and hover-position stability against real history APIs and backend WebSocket events. Both 1440px desktop and 390px mobile layouts passed; no uncaught page errors. Workspace-tab and language state were isolated to avoid changing active clients. Reload-aborted requests from the previous document were recorded separately.
• Local full run: 609/610 test files and 9,087/9,096 tests passed; failures were confined to the unchanged forge-page.test.tsx. Running that file on the untouched upstream baseline also failed (6/64). The latest PR revision's clean CI full suite passed.