feat: allow side elements to opt out of automatic expansion

[minwookshin]

Addresses #2932.

A new or updated side element currently reopens a panel the user has closed. Add a keyword-only auto_expand=False option to elements, including CustomElement, and carry it as the optional autoExpand live event hint.

The frontend keeps a closed panel closed for opted-out arrivals, updates an already open panel, and still opens it when the user clicks the message reference. Existing callers keep automatic expansion by default. When no side elements remain, cleanup only closes a view still displaying a tracked side element; sidebars opened through ElementSidebar.set_title or set_elements retain their contents, title and key across unrelated inline/page updates.

Before	After	Why
Every side-element update can reopen the panel	Individual arrivals can opt out	Lets applications respect a user's reading flow
Explicit reference clicks open side content	Explicit clicks still open opted-out elements	Preserves direct user control while suppressing automatic opening

Validation: All 938 backend tests on Python 3.13 and all 44 frontend tests pass. The seven new sidebar cases include six preservation regressions that fail before the cleanup fix, including a replacement reusing a tracked ID, and one explicit opt-out element cleanup case. Six API compatibility cases also pass on Python 3.10, covering Text, File and CustomElement with both values of auto_expand. The repository pre-commit checks, including full backend mypy, source/test lint and frontend/react-client typechecks, pass. Repository-wide ESLint/format checks and the embedded copilot build also pass. The production frontend Vite build passes with NODE_OPTIONS=--max-old-space-size=4096; the default 2GB Node heap was insufficient on the validation machine. Existing sourcemap and chunk-size warnings remain. Chrome verification against the real backend covers default opening, quiet Text and CustomElement arrivals, and explicit opening of both kinds. Documentation includes the API and persistence boundary.

The hint is intentionally not persisted; restored historical threads keep their current display behavior. Apply the option to each new or updated element that should remain quiet.

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Summary by cubic

Fixes the side panel reopening on every side-element update (issue #2932). Elements can now opt out with auto_expand=False and remain reachable only through their message reference.

• auto_expand applies to Element and CustomElement and defaults to True.
• The hint is live-only and not persisted, so restored historical threads keep their current behavior; it also survives the REST element update endpoint.
• Opted-out elements keep a closed panel closed on arrival or update; explicit reference clicks still open them.
• Programmatically opened sidebars are preserved when inline/page elements arrive, when tracked content is replaced, or across thread navigation; clearing is limited to views showing tracked side elements.

^{Written for commit 865ebcc. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 7 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[minwookshin]

@codex review

[cubic-dev-ai]

All reported issues were addressed across 2 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 4 files (changes from recent commits).

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>

[minwookshin]

Preserved autoExpand on custom-element updates and added a navigation regression check. Focused backend/frontend tests pass locally; the pre-commit hook could not run here because uv is unavailable.

[cubic-dev-ai]

All reported issues were addressed across 3 files (changes from recent commits).

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>

[minwookshin]

Tracked side views opened from shared threads and added a regression test through the REST update route. The affected frontend/backend suites, formatting, lint, and typecheck pass locally.