Hand the timeline's date comparison off to the Comparison plugin

[sandesh-sp]

Summary

Timeline-side groundwork for comparing two dates on the map. The timeline's Compare date action becomes real, DateSelector becomes reusable outside the timeline header, and the picker is corrected to work in UTC.

What's here

Compare hand-off. Clicking Compare date announces plugin:comparison:startWithDates on the mmgisAPI bus. The timeline knows the Comparison plugin only by the name of that event — a mission that doesn't load the plugin is one where nobody listens, not one where the timeline breaks.

DateSelector becomes embeddable. Three additions let another layout reuse the component as-is:

• className — appended to the root, so an embedder can position it
• placeholder — wording shown in place of a formatted date, so a date the user hasn't picked yet reads as an invitation while the calendar still opens around selectedDate
• onCompareClick — the Compare date action and its divider are drawn only when a handler is supplied

UTC correctness. MMGIS stores every instant in UTC, but the picker was formatting and parsing in local time. A viewer west of Greenwich saw — and committed — the wrong day. Every moment(...) in DateSelector and DayCalendar is now moment.utc(...).

Stylesheet split. DateSelector.css and FloatingPopover.css move out of Timeline.css and are imported by the components that own them, so the pieces travel with their directories. lib/css-imports.d.ts declares plain .css imports, mirroring the scss-imports.d.ts other plugins' libs carry.

Timeline window on the bus. mmgisGetTimeStart / mmgisGetTimeEnd / mmgisGetTimeCurrent expose the timeline's window to tools that need to read it without owning it.

Tests

src/essence/Tools/Timeline/__tests__/ — 10 cases, 2 files:

• DateSelector.spec.tsx pins the process timezone to America/Los_Angeles before importing the component. On a UTC CI host, local and UTC formatting are identical and a local-time regression would pass; under this zone it surfaces as a wrong day or a shifted clock.
• TimelineAdapter.spec.tsx covers that the action is offered and that clicking it puts the event on the bus.

npx vitest run src/essence/Tools/Timeline/__tests__
 Test Files  2 passed (2)
      Tests  10 passed (10)

Notes for review

This is the timeline half only. The map-side comparison work — side-by-side layout and per-side date pins — lives on feat/compare-tool-core and lands separately; nothing here depends on it. The emitted event has no listener until that branch merges, which is the intended failure mode.