ui(cards): per-scene quick actions — duplicate, move to another episode

[stultus]

Where: `src/lib/components/SceneCardsView.svelte`

The only quick action on a scene card today is Delete (the trash icon in the header). Two more would land naturally:

1. Duplicate scene — clone the card's authored fields plus the underlying `scene_heading` block. Writers often plan parallel scenes (different POVs of the same event) and currently have to copy-paste in the editor and re-author the cards.

2. Move to another episode (series only) — pop a small menu listing the other episodes; selecting one moves the scene's ProseMirror node + its scene_card data from the current episode to the chosen one. Recomputes scene_index on both episodes.

Suggested shape:

• A … overflow icon next to the existing trash icon, opening a small menu:
  • Duplicate
  • Move to episode → submenu with the series' other episodes
  • (Future) Convert to Action / Convert to Note
• Trash stays as the explicit destructive primary; the overflow holds reversible-ish actions.

"Move to episode" is the bigger value-add — for series writers re-planning episode boundaries mid-draft, dragging a scene from Ep 4 to Ep 3 is a real authoring move. Currently it requires manual cut/paste in the editor and recreating the card metadata.

Pairs with #156 (IDE-style explorer, drag-between-folders is a follow-up there) — the same data move, exposed in two surfaces.

[stultus]

Duplicate shipped in fd8309d. Move-to-episode deferred — needs cross-episode data movement plus a proper submenu, will be its own focused issue.