What

Tenth PR in the flat inspector stack (see #2120 for the foundation and full stack list). Removes group pinning entirely — the pin button, PinnedGroupRow, PinnedZoneDivider, the usePersistedPinnedGroups hook (deleted outright, including its test file), and the pinnedGroupsByElementType field from studioUiPreferences.ts.

Stack: #2120 → ... → #2134 → #2135 (this).

Why

Explicit product decision: pinning added complexity without being worth keeping. Removing it also resolves the one known follow-up flagged in the previous PR (#2134) — an oversized pinned zone could silently clip content with no scrollbar, since there's no scroll fallback in the new fixed-header layout. With no pinned zone at all, that failure mode can't occur.

How

• PropertyPanelFlat.tsx's group list no longer splits into pinned/unpinned — the beforeOpen/openGroup/afterOpen split (from refactor(studio): scroll only the open group's body, keep headers fixed #2134) now operates directly on the full group list.
• FlatGroupHeader no longer accepts isPinned/onTogglePin; the pin button is gone from its button row.
• PinnedGroupRow and PinnedZoneDivider deleted from propertyPanelFlatPrimitives.tsx.
• usePersistedPinnedGroups.ts and its test file deleted entirely.
• pinnedGroupsByElementType removed from StudioUiPreferences and its read/write validation.

Test plan

• All pin-specific tests removed (not left dangling/skipped) across PropertyPanel.test.tsx, propertyPanelFlatPrimitives.test.tsx, and studioUiPreferences.test.ts.
• Verified live that no pin UI remains anywhere in the inspector and normal group open/close still works correctly.
• Full monorepo suite green (1578 tests, 0 failures); oxlint/oxfmt/fallow clean.
• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)