feat(scenes): render the Scene panel's controls from the store

SceneControls no longer takes element handles or writes the Scene panel's
button states, PLAY label, status line, progress bar or runtime chip: it
presents them to the gev-scenes store, and the panel's buttons, scene
select and IMPORT chooser call its actions there. The chooser opens only
while an owner presents the panel and is cleared once its import settles.
On destroy the panel keeps its last status and buttons, without shots or
the runtime chip, as before.

docs/VUE-MIGRATION.md

@@ -91,7 +91,9 @@ a feature supplying only its accent.
    the rail cards, card blocks and timeline (`src/ui/rail/`) render through
    Vue's synchronous `render()` behind `createRailCards()`,
    `createRailCardBlocks()` and `createRailTimeline()`. Order: Scenes (done: `SceneOptions` and
-   `SceneShotList` over the `gev-scenes` store), Weather (done: the owner
+   `SceneShotList` over the `gev-scenes` store; the panel's buttons, status
+   line, progress bar and the runtime chip render the panel SceneControls
+   presents there, and the buttons and file chooser call its actions), Weather (done: the owner
    renders `WeatherReadout` from the view it computes), Recent imagery
    (done: `RecentImageryReadout` and its day cards, the same way),
    Street level (done: `StreetLevelPanel` renders the `gev-street-level`