feat: add As-Of time-travel UI for viewing historical snapshots

[bhudevbhanpuriya]

Introduces a full As-Of (time-travel) feature for the map/datastream UI, letting users view the SensorThings graph as it existed at a past point in time instead of live.

State & data layer:

• AsOfContext/AsOfProvider (context/AsOfContext.tsx): global asOfDate/isSnapshot state, exposed via useAsOf().
• asOfAdapter.ts: single AsOfAdapter interface (resolveThing, fetchCommits) with two implementations selected by NEXT_PUBLIC_VERSIONING_ENABLED:
  • mockAdapter: resolves state from local-dummy-things.ts, for demo/dev use with no backend versioning support.
  • apiAdapter: calls the real backend via Things(id)?$as_of= and Things(id)/Commit.
• useAsOfThing / useAsOfCommits: hooks wrapping the adapter, exposing the resolved snapshot Thing, existence state (exists/not-yet-created/deleted), and commit history for a Thing.

UI components (features/as-of/components/):

• AsOfNavbarButton + AsOfDropdown: navbar pill (Live / snapshot chip) and a calendar+time picker with quick-range presets, with a future-date guard.
• AsOfBanner: page-wide banner shown while in snapshot mode.
• TimelineScrubber: bottom-fixed slider across [earliest commit, now] with clickable commit tick marks.
• AsOfExistenceDialog: dismissible floating alert with a "jump to nearest valid date" action when asOfDate falls outside a Thing's known lifetime.

Wiring into the app:

• layout.tsx: wraps the app in AsOfProvider, renders AsOfBanner.
• Navbar.tsx: adds AsOfNavbarButton, reworks the navbar into a 3-column grid to fit it.
• Home.tsx: resolves the selected Thing's snapshot via useAsOfThing and feeds the resolved snapshotThing (not the live thing) into DatastreamTable; renders TimelineScrubber and AsOfExistenceDialog; re-arms the existence dialog only on a genuine transition into an invalid state, not on every scrubber tick.
• DatastreamTable.tsx: read-only banner and hides Edit/Delete/New/ Import actions while isSnapshot is true.
• LeafletMap.tsx / leafletDraw.ts / leafletCluster.ts: snapshot markers get an amber date-pill divIcon (createThingMarkerDivIcon); MapContextMenu hides "New"/"Import from file" during snapshot mode so the map stays read-only like the table.
• ChartModal.tsx / ObservationGraph.tsx / observationGraphOptions.ts: clamps the date-range picker to asOfDate, draws a dashed snapshot marker line on the chart.
• observations.ts: appends $as_of to the observations request when in snapshot mode.
• useChartState.ts: namespaces the observation cache by asOfDate so live and snapshot results never collide.
• useDataSourcesSync.ts: injects the local dummy stations onto the map only when NEXT_PUBLIC_VERSIONING_ENABLED is false (mock mode).

Known limitations

• Production Docker build isn't a working target for this project yet
  (pre-existing, see README.GSoC.md TODO) - the NEXT_PUBLIC_VERSIONING_ENABLED
  fix in this PR only covers local dev (dev-docker-compose.yaml).
• apiAdapter.fetchCommits only returns the backend's single creation commit
  today (no plural /Commits endpoint exists yet), so the timeline scrubber's
  tick marks are limited when running against a real backend.

[bhudevbhanpuriya]

Update — follow-up commit (0b06278)

Refines the As-Of chart behavior and fixes an existence-state bug found while
testing against a live istSOS4 backend (VERSIONING=1). No new files; touches
5 existing ones. Verified with tsc --noEmit (clean) and against the running
backend API.

Chart in snapshot mode

• The observations chart now reacts to asOfDate changes (scrubber, calendar,
  and the existence-dialog "jump to nearest data") instead of staying stuck on a
  stale window from a previous snapshot. The refetch is debounced on the trailing
  edge so continuous scrubbing fires a single request.
• Default window is strictly [asOfDate − 7d, asOfDate]; the persisted range and
  observation cache are cleared on any snapshot/live context switch.
• Refetches the whole current selection (all things / observed properties) and
  preserves an active comparison across snapshot changes, not just the primary
  datastream.

Empty-window feedback

• When the snapshot window has no data, the chart shows an informational warning
  describing where the data actually is (before/after the range) — without ever
  mutating the as-of state. This is expected with the demo data, where measurements
  (phenomenonTime ≈ 2020) were committed to the DB in 2026, so a recent snapshot's
  7-day window is legitimately empty until you pan the picker back.

Chart rendering

• Pins the x-axis to the queried window in snapshot mode, so sparse/empty data still
  renders the full range and the snapshot marker line can't stretch the axis.
• Draws the snapshot marker line once (primary series) instead of once per series.

Existence-state fix (asOfAdapter.ts)

• "Not-yet-created vs deleted" is now derived from the Thing's commit history
  (transaction time — the axis $as_of filters on) instead of phenomenonTime,
  which can be years off from when the row was written. Previously a snapshot that
  predated a Thing's creation was mislabeled "deleted" with a wrong jump target.
• fetchCommits now reads the plural /Things(id)/Commits collection for full
  history (falling back to the singular /Commit).

[bhudevbhanpuriya]

rebased on latest main, no conflicts, and moved all UI strings to i18n

[bhudevbhanpuriya]

Update - two new commits since this was last looked at.

1. Removed my local testing changes.
While developing I had turned off authorization in data-sources.json and
committed a generated build file (tsconfig.tsbuildinfo). Both are now reverted.
They were never part of the feature.

2. Added translations.
All the text in the As-Of UI was written directly in English, but the rest of the
app reads its text from the translation files so it can switch between English
and Italian. I moved every piece of visible text — buttons, labels, warnings,
hover text — into en/translation.json and it/translation.json, the same way
#20 did for the observation graph. Month and weekday names in the calendar are
translated as well.

What I checked: the TypeScript build passes, the English and Italian files have
exactly the same list of keys, and every piece of text the code asks for exists
in both files. I checked this by reading the code, not by clicking through the
app, so a quick look at the Italian version would be worth it.

There are also 4 warnings in these files that the linter flags. They were there
before these commits — happy to fix them here or leave them for later, your call.

No conflicts with GSoC_2026-TimeTravel. One question: that branch is 8 commits
behind main - would you rather this PR pointed at main?

Next I'm starting the $from_to UI, which reuses these same components, so
feedback on the shared parts here would help before I build on top of them.