Qt-removal R7.5e: Collapse All / Expand All

[dovvnloading]

Fifth R7.5 parity sub-increment, after R7.5a (#147), R7.5b (#148/#149/#150), R7.5c (#151), and R7.5d (#152/#153).

Problem

Legacy exposes "Collapse All Nodes"/"Expand All Nodes" commands (graphlink_window_navigation.py:12-13,31-39) that iterate only chat/conversation/html_view nodes and call set_collapsed(bool) on each. The SPA has no equivalent — the per-node primitive existed (SceneNode.is_collapsed, the setChatCollapsed intent, real collapse buttons on ChatNodeView/ConversationNodeView/HtmlNodeView), but no bulk operation.

Change

• backend/canvas.py: SceneDocument.set_all_conversational_collapsed(collapsed: bool) — restricted to kind in ("chat", "conversation", "html"). Frame/container nodes are deliberately excluded: their is_collapsed drives derived geometry via _recompute_group_bounds, which the bulk setter never calls, so including them would desync is_collapsed from group_width/group_height.
• Two new intents, collapseAllNodes/expandAllNodes, each performing one document mutation followed by exactly one publish_scene() call — matching the existing organize_nodes handler's shape, never one publish per node.
• web_ui/src/app/canvas/sceneStore.ts: two thin fire-and-forget methods sending the new intents.
• web_ui/src/app/chrome/commands.ts: two command-palette entries with legacy's exact names ("Collapse All Nodes"/"Expand All Nodes") and aliases (fold all/unfold all), gated on scene content rather than selection — matching legacy's own bool(scene._all_conversational_nodes()) predicate.

Test plan

• 885 backend tests (5 new) — kind-scoping correctness, frame-not-clobbered-by-expand-all, both intents' single-publish behavior, empty-scene safety
• 1208 frontend tests (4 new), 0 lint errors, typecheck/build clean
• Burn-down gate unchanged at 152/84/68
• Adversarial review of the diff — no defects found
• Live-verified against a real backend: seeded a 5-node scene (chat/conversation/html wrapped in a frame, plus an unwrapped code node), ran "Collapse All Nodes" via the real command palette and confirmed only chat/conversation/html flipped to isCollapsed: true; manually collapsed the frame, ran "Expand All Nodes", and confirmed the frame stayed collapsed while the three conversational nodes expanded