refactor(studio): remove section pinning from the flat inspector

[vanceingalls]

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)

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(studio): resolve flat-inspector review defects #2225
• fix(studio): debounce flat slider drag commits to prevent rapid-fire writes #2190
• fix(studio): support drag on FlatSlider, not just click-to-set #2186
• fix(studio): widen FlatSlider's click/drag hit area vertically #2142
• feat(studio): animate flat inspector group expand/collapse #2136
• refactor(studio): remove section pinning from the flat inspector #2135 👈 (View in Graphite)
• refactor(studio): scroll only the open group's body, keep headers fixed #2134
• fix(studio): Style/Grade/Text parity gaps found in legacy-retirement audit #2127 : 2 other dependent PRs (#2128 , #2129 )
• fix(studio): retire legacy Style and Grade sections from flat inspector #2126
• feat(studio): flat inspector — persisted pinning + multi-field Text #2125
• feat(studio): flat inspector — Grade (color grading) group #2124
• feat(studio): flat inspector — Media group #2123
• feat(studio): flat inspector — Layout + Motion groups #2122
• feat(studio): flat inspector — Style group #2121
• feat(studio): flat inspector foundation + Text group #2120
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[miga-heygen]

Reviewed as part of the 15-PR edit-panel redesign stack. Full stack review posted on #2120. No blockers on this PR. — Miga

[miguel-heygen]

Final current-head pass: 1fea656. Reviewed the stack boundaries and current diff against the flat-inspector contracts; required checks have no failures/pending checks and no unresolved review threads remain. No new drift found; residual notes are non-blocking.

[miguel-heygen]

Superseding earlier approval: the latest max-review pass found confirmed correctness issues in this flat-inspector stack (identity/selector-index state, hide-all write races, timing inference, slider pointer/keyboard/reset semantics, and duplicate React keys). Hold merge and require fixes plus re-review on the current stack head.