UI improvements: split screen, faster profile, smoother tiles

[mgovorcin]

UI improvements: split screen, faster profile, smoother tiles

A grab-bag of frontend + backend polish on top of main, packaged together so the underlying refactors (custom Leaflet panes, two-slot tile model, shared RasterTileLayer props) only land once.

New features

• Split-screen comparison. Toggle from the right toolbar to render a second dataset in a clipped pane with a draggable divider. A dedicated "Right Layer" sidebar section gives it its own dataset, colormap, vmin/vmax, opacity, time-step slider, and histogram. The pixel-inspector reads the correct dataset and time index based on which side of the divider the cursor is on (tooltip prefixed L:/R:).
• Stacked basemaps. Basemap popover now exposes two slots — Layer 1 (bottom) and Layer 2 (top) — each with its own opacity slider and a Swap-order button. Lets you combine, e.g., satellite imagery underneath a labelled street map for context, with on-the-fly blending.
• Coordinate grid. Plain / zebra-frame graticule, sitting above tiles but below markers (custom Leaflet pane at z-index 450). Lines render with a white halo so they stay visible on dark satellite basemaps.
• Map annotations. Click the pen-to-square button to enter annotation mode, then click anywhere on the map to drop a labelled marker with custom text, color, and font size. Markers are draggable; click an existing one to edit text/color/size or delete it. Persists across map operations as part of state.annotations.
• Pixel value on hover. Eye-dropper button on the right toolbar toggles a hover inspector that follows the cursor and reads the raster value (with dataset unit) at the mouse position from the backend. In split-screen mode it queries the correct dataset and time index per side and prefixes the tooltip L:/R:. Issue ticket: Hover to show value #35
• Two-slot tile loading. New tile URLs load behind the active layer at opacity 0 and atomically swap on load; no more mixed-tile seam artifacts when scrubbing time / colormap / vmin-vmax. A bottom-right spinner reflects in-flight tile counts across basemaps + raster.
• Screenshot export. Camera icon (📷) added to the chart panel, colorbar, and LOS indicator — clicking it downloads a transparent-background PNG of just that element. A black/white font toggle on the colorbar and LOS lets labels read against any background.
• Faster profile extraction. Binned-median mode now vectorizes all per-bin pyproj.transform + xarray.sel(method='nearest') calls into a single batched read — 20–100× faster on zarr-backed datasets. The start→end gradient line was restored in binned mode (was hidden behind showLine: false). X button on the panel now closes it instead of just clearing data. Awkward shape was addressed before: "radius" round profile is strangely shaped #37
• Pre-computed histograms. tifs-to-geozarr embeds per-time-step histogram stats (min/max/percentiles/bins) as a bowser_histogram zarr attr; the /histogram endpoint returns cached stats instantly. Falls back to a full compute for older stores.

Smaller polish

• Reference + point-picking buttons moved from the right toolbar to the left, so the right one stays focused on visualization.
• Toolbar background uses CSS variables → follows the dark/light theme.
• Reference marker visibility decoupled from refEnabled (new refMarkerVisible state + toggle).
• Complex-mode useEffect no longer overwrites the localStorage-restored vmin/vmax when switching datasets.
• Browser-side histogram cache keyed by dataset:timeIndex (no refetch when scrubbing).
• Toolbar-hide now also hides the Points panel (lifted toolbarsVisible to App.tsx).

Backend fixes

• Layer-mask time-index clamping. When a mask dataset has fewer time steps than the main dataset, the tile endpoint now isels min(time_idx, size-1) (falling back to 0 when there's no time context) instead of raising IndexError.

Notes for reviewers

• RasterTileLayer was generalized to accept pane and overrideDataset/colormap/vmin/vmax/opacity/timeIndex props so the split-screen right layer reuses the same fetch/swap pipeline. No duplicated tile logic.
• New panes (splitLeft, splitRight, graticule) are created synchronously in the render function of their respective components — useEffect / useLayoutEffect ran after React-Leaflet's addLayer, causing getPane() is undefined crashes. This is the only reliable ordering with react-leaflet ^4.2.
• clip-path for the split panes uses polygon() with absolute pixel values derived from the map-pane's CSS translate (updated on move/zoom/resize); percentage-based inset() was relative to the pane's full tile-buffer extent, not the viewport.

Testing

Split-screen