Fix dashboard session resume terminal remount

[X1pheR]

Summary

• keep dashboard chat on the requested ?resume= target instead of rewriting it after the PTY has already started
• key the xterm host by PTY identity and clear stale xterm DOM during cleanup before remounting
• avoid WebGL for embedded dashboard chat because resumed buffers can blank after renderer/layout settling
• skip sidecar/event sockets when the sidebar is rendered as a model/session rail with showTools=false

Testing

• npm run typecheck --workspace web
• npm run build --workspace web
• npx eslint src/pages/ChatPage.tsx src/components/ChatSidebar.tsx (passes with existing warnings only)
• manual verification in the dashboard: resuming sessions from /sessions / right rail no longer turns the terminal black after the delayed refresh; text remains visible

[teknium1]

Thanks for isolating the dashboard remount race. The current code still has the triggering sequence: web/src/pages/ChatPage.tsx:347-356 asynchronously rewrites ?resume=, while the PTY effect is keyed by channel/resumeParam at web/src/pages/ChatPage.tsx:1168.

Problems

• Removing the lookup drops the intentional latest-child behavior added in b12a5a72b (Follow latest child session on dashboard resume). The comment says callers should resolve before navigation, but this PR does not add such a caller-side resolution.
• The showTools branches are stale against current main: ChatSidebarProps has no such prop at web/src/components/ChatSidebar.tsx:73-80, and both ChatPage call sites omit it at web/src/pages/ChatPage.tsx:1364-1369 and 1484-1489.
• There is no regression coverage for a delayed descendant lookup racing an already-connected PTY.

Suggested changes

• Preserve descendant canonicalisation before the live terminal mounts, and add a delayed-resolution remount regression test.
• Rework or omit the stale showTools portion against current ChatSidebar's actual API.

Automated hermes-sweeper review.

[teknium1]

Current main no longer declares or passes showTools: ChatSidebarProps is at current web/src/components/ChatSidebar.tsx:73-80, and both ChatPage call sites omit it. This branch must be reworked against the current sidebar API before salvage; a direct cherry-pick will not typecheck.