feat(ui): render the rail timeline as a component behind its builder

The observed-history transport is RailTimeline in src/ui/rail/, rendered
by createRailTimeline() with the same update and destroy API. Drag preview,
the coalesced 150 ms commit, release, cancel and blur behave as before; the
slider keeps the ticks it is dragged over, an update from inside a preview
cannot move the handle back, and readouts follow the handle within the same
task. utcClock moves to its own module and is still exported here.

docs/VUE-MIGRATION.md

@@ -78,9 +78,9 @@ They contain no application state.
    `v-for`, because re-rendering it would bring back the English; component
    text uses `t()` instead. Shared builders become components first and
    keep their imperative entry points until their owners are components:
-   the rail cards and card blocks (`src/ui/rail/`) render through Vue's
-   synchronous `render()` behind `createRailCards()` and
-   `createRailCardBlocks()`. Order: Scenes (done: `SceneOptions` and
+   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, Recent imagery,
    Street level, CCTV, Layers, Display, Context and Radio, the command dock,
    the phone shell, then Cockpit. Layers that write into the right rail