feat(graph): completed nodes orbit as stars around the task sphere

[diogobaltazar]

Description

When a node is marked complete, it escapes the main sphere and relocates to an outer "stellar" orbit — smaller, luminous, star-like. Edges from completed nodes to their still-incomplete neighbours remain visible with the existing glow effect, preserving the sense of connection. The overall impression is a planet (the active task sphere) surrounded by a constellation of completed work.

Visual behaviour

• Position: Completed nodes move to an outer invisible level beyond the sphere surface. The sphere supports as many outer levels as the graph has branch-depth levels — so a graph 4 levels deep gets 4 possible stellar rings, one per depth stratum.
• Size: Completed nodes render at reduced size (suggested: ~40–50% of a normal node).
• Appearance: Each completed node glows with a star-like shine — a bright core with a soft radial bloom, using the existing accent/glow token palette.
• Edges: Edges from a completed node to any of its incomplete neighbours are still drawn, using the current edge glow/pulse effect unchanged.
• Inactive nodes: Nodes not yet completed remain on the sphere exactly as today.

Acceptance criteria

• Marking a node complete triggers an animated transition: it moves from its sphere position to the outer stellar level corresponding to its branch depth, shrinks to the reduced size, and adopts the star appearance.
• Edges between a completed node and its incomplete neighbours continue to render with the existing effect.
• Marking a node incomplete reverses the transition — it returns to the sphere.
• The layout computation in graph.js is updated to place completed nodes on outer levels keyed by branch depth.
• The star glow is implemented in App.css / inline styles in App.jsx without introducing new runtime dependencies.
• Performance is acceptable with up to ~200 nodes (no frame-rate regression on the existing sphere animation).