feat(context-menu): add right-click and long-press actions (#51)

[DevamPatel22]

Summary\n- add context menu actions on deck cards\n- add context menu actions on deck detail list items\n- support desktop right-click and mobile long-press entry points\n\nCloses #51

[Pairadux]

A few things to address:

Duplicated menu construction - _showContextMenuAt() in DeckDetailScreen and _showContextMenu() in DeckCard build identical popup menu items separately. Define them once so adding new actions later only requires one change.

Inconsistent pattern - Deck context menus: DeckCard shows the menu, then fires onContextMenuAction. Card context menus: DeckDetailScreen shows the menu and handles the action directly. Pick one approach for both.

Gesture conflicts - Wrapping InkWell inside GestureDetector can cause issues (ink splash may fire before the context menu appears). Consider using a single gesture layer instead.

onLongPress silently dropped - When onContextMenuAction is set, the onLongPress callback gets nulled out. Not a bug right now, but could surprise future callers.

Missing from DeckListScreen - Right-click and long-press context menus only work inside a deck (on DeckDetailScreen). They don't work on the main deck list screen. If that's intentional for this PR, please open a follow-up issue to add it there too.

Good work overall. Clean enum, proper mounted checks, and the disabled "Move" placeholder is a nice touch.

[Pairadux]

If adding to core is needed for first thing, let me know, but its probably not a problem