feat(web): close remaining editor, layout and persistence gaps

- Area headers get a per-area editor switcher backed by a shared
  AreaEditorContext; Preferences can hide it.
- Animation camera area is a real navigable viewport aligned to the
  scene camera; Graph Editor adds channel selection, keyframe drag/
  insert/delete and interpolation.
- Geometry Nodes node locations round-trip through the native writer
  and reader, so saved graphs keep their layout.
- Texture Paint can make linked library images local via a new native
  makeImageLocal command before packing strokes.
- Compositor IMAGE and Render Layers nodes now persist to Main and
  execute against packed images / the last PBR render.
- Spreadsheet row cap raised to 250k; Outliner gains Ctrl+A and arrow/
  Home/End navigation.

Co-Authored-By: Claude Code <noreply@anthropic.com>

web/protocol/geometry-nodes.ts

@@ -89,6 +89,8 @@ export interface GeometryNodeIR {
   sockets: GeometryNodeSocketIR[];
   groupTreeId?: string | null;
   properties?: Record<string, boolean | number | string | number[]>;
+  /** Canvas position in Blender node-space units. */
+  location?: { x: number; y: number };
 }
 
 export interface GeometryNodeLinkIR {
@@ -421,6 +423,13 @@ export function parseGeometryNodeGraph(value: unknown): GeometryNodeGraphIR {
       if (record(socket)) socketIds.add(socket.id as string);
     });
     if (node.groupTreeId !== undefined && node.groupTreeId !== null) boundedText(node.groupTreeId, `nodes[${index}].groupTreeId`, GEOMETRY_NODE_GRAPH_BUDGET.maxIdentifierBytes);
+    if (node.location !== undefined) {
+      if (!record(node.location) || typeof node.location.x !== "number" || typeof node.location.y !== "number" ||
+          !Number.isFinite(node.location.x) || !Number.isFinite(node.location.y) ||
+          Math.abs(node.location.x) > 1_000_000 || Math.abs(node.location.y) > 1_000_000) {
+        throw new GeometryNodeGraphError("GN_INVALID_GRAPH", `nodes[${index}].location is invalid`, `nodes[${index}].location`);
+      }
+    }
     if (node.properties !== undefined) {
       if (!record(node.properties) || Object.keys(node.properties).length > 64) throw new GeometryNodeGraphError("GN_INVALID_GRAPH", `nodes[${index}].properties is invalid`, `nodes[${index}].properties`);
       for (const [name, property] of Object.entries(node.properties)) {