UI-A3 — Smoother transitions (dialogs, dropdowns, tree, tabs)

[ZhuchkaTriplesix]

Epic: #170
Depends on: #173, #171 (UI-A1, UI-A2)

Source: docs/motion-and-high-refresh.md §4.4

Goal

Upgrade interaction choreography beyond token swap: enter/exit transitions, expand/collapse height, and content cross-fades.

Scope

Area	Change
Dialogs (app_dialog.dart)	Retune blur+scale to standard/enter; backdrop and card share one curve
Dropdowns (querya_dropdown.dart)	Add menu enter scale+fade (currently hover-only)
Connection tree (connections_panel_pg_tree.dart + siblings)	AnimatedSize for expand/collapse; keep chevron rotation
Workspace tabs (workspace_panel.dart)	AnimatedSwitcher cross-fade on content switch
Hover rows	Already tokenized in A2; verify feel at 120 Hz

Optional if time: subtle first-paint fade on result grid rows (no per-scroll cost).

Acceptance Criteria

• Tree nodes expand/collapse with height animation (no pop-in).
• Dropdown menu opens with fade+scale enter.
• Workspace panel content cross-fades on tab change.
• Dialog open/close feels smooth at 120 Hz (manual check).
• Layout tests green; no overflow regressions.

Tests

• Widget tests for AnimatedSize tree node (expanded/collapsed state).
• Existing layout tests unchanged in final geometry.

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