feat: add collapse/expand all buttons for collection items

[makstech]

Adds per-section collapse/expand all buttons for items within collections and the ungrouped items section.

• Collapse/expand buttons appear in each collection header when the collection contains 2+ items
• Ungrouped items section is now a collapsible group with a caret toggle, matching collection header styling
• Only the relevant button is shown: expand when items are collapsed, collapse when expanded
• Buttons are hidden when the section itself is collapsed
• Top-level "Expand All" / "Collapse All" now also affects the ungrouped section

Closes #3530

Summary by CodeRabbit

• New Features
  • Ungrouped items now have a collapsible header with visual caret and can be shown/hidden.
  • Per-section "collapse all" / "expand all" controls added for collections and for ungrouped items.
  • Batch collapse/expand operations added to toggle multiple sections at once for faster management.
  • Global expand/collapse controls now include the ungrouped section.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 3b391cd5-497c-462b-9926-862271a0a54c

📥 Commits

Reviewing files that changed from the base of the PR and between 04f2f6c and 3113b90.

📒 Files selected for processing (1)

• webui/src/Variables/CustomVariablesList.tsx

🚧 Files skipped from review as they are similar to previous changes (1)

• webui/src/Variables/CustomVariablesList.tsx

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📝 Walkthrough

Walkthrough

Introduces collapsible UI and batch item-collapse controls for ungrouped collection items: a new UngroupedSection with UNGROUPED_PANEL_ID, an item-level CollectionItemsCollapseButtons component, and a setMultipleCollapsed helper for atomic batch collapse/expand updates.

Changes

Cohort / File(s)	Summary
Ungrouped section & panel id webui/src/Components/CollectionsNestingTable/CollectionsNestingTable.tsx	Replaced inline ungrouped rendering with a new UngroupedSection subcomponent and exported UNGROUPED_PANEL_ID. Adds collapsible header/caret UI and conditional rendering of collapse controls when applicable.
Item-level collapse controls webui/src/Components/CollectionsNestingTable/CollectionsNestingTableGroupsList.tsx	Added CollectionItemsCollapseButtons (observer) to batch-expand or batch-collapse multiple item panels; integrates with collection item rendering to show controls when a collection has >1 item.
Batch collapse API webui/src/Helpers/CollapseHelper.tsx	Extended PanelCollapseHelper interface and store with setMultipleCollapsed(panelIds: string[], collapsed: boolean) to update many panel states in a single MobX action and persist once.
Top-level expand/collapse integration webui/src/Variables/CustomVariablesList.tsx	Included UNGROUPED_PANEL_ID in the set of panel IDs used by top-level expand/collapse buttons so ungrouped items participate in global expand/collapse actions.

Poem

📦 Ungrouped lines fold and hide,
Caret twinkles, toggles wide,
Batch a bunch with one swift click,
Panels shrink and then unstick,
Debugging dances, tidy and bright ✨

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main feature: adding collapse/expand-all buttons for collection items, which is the core objective of this PR.
Linked Issues check	✅ Passed	The PR fully implements issue #3530 requirements: per-collection collapse/expand buttons (for 2+ items), ungrouped section collapsibility, context-aware button display, and integration with top-level Collapse All behavior.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to implementing the collapse/expand functionality for collections and ungrouped items; no unrelated modifications detected.

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 989c88de-acb9-405f-9949-6d5fb0dc6c29

📥 Commits

Reviewing files that changed from the base of the PR and between 0a7ada7 and 04f2f6c.

📒 Files selected for processing (4)

• webui/src/Components/CollectionsNestingTable/CollectionsNestingTable.tsx
• webui/src/Components/CollectionsNestingTable/CollectionsNestingTableGroupsList.tsx
• webui/src/Helpers/CollapseHelper.tsx
• webui/src/Variables/CustomVariablesList.tsx