Canvas: note cards (add, drag, persist, open)

[KrasForge]

Summary

Place note cards on the canvas: add existing notes from a picker, drag cards to position them, persist their layout, and open a note by double-clicking its card.

Part of #342. Depends on the canvas scaffold and pan/zoom surface.

Behavior

• Add existing notes to the canvas from a picker; render each as a card.
• Drag a card to reposition it within the canvas transform.
• Double-click a card to open its note (switch to the notes view with that note selected).
• Card positions persist client-side, consistent with how the note tree and plugin install state are stored.

Implementation notes

• Read notes from data.notes (the WorkspaceViewProps the runtime passes to a view).
• Persist card positions to localStorage, keyed per canvas.
• Reuse the node-drag handling from GraphView for card dragging under the view {x,y,k} transform.
• Cross-view navigation to open a note: switch the active view to notes with the target note selected, matching how nav and selection are wired in Workspace.tsx and NotesView.tsx.

Acceptance criteria

• Cards can be added from a picker and dragged; their positions survive a reload.
• Double-clicking a card opens the note in the notes view with it selected.
• Renders correctly in light and dark themes.