feat: seamless domain border overlay

[vincentvdwal]

Draws the boundary of each sub-domain of a seamless composite: fading in two
zoom levels before the sub-domain becomes active, highlighting it once it is,
and dropping regional borders past their maxForecastHours — matching the
composite's own fallback to a coarser model.

• updateSeamlessBorderLayer / resetSeamlessBorderLayer in src/lib/layers.ts
• a settings toggle to turn the borders off

Stacked on #399. Verified: npm run check 0 errors, 100 tests pass.

[cloudflare-workers-and-pages]

Deploying maps with    Cloudflare Pages

Latest commit:	25a022c
Status:	✅  Deploy successful!
Preview URL:	https://d9516ce5.maps-5aj.pages.dev
Branch Preview URL:	https://seamless-domain-mode.maps-5aj.pages.dev

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[Copilot]

Pull request overview

Adds support for “seamless” (multi-layer) domains by resolving an appropriate concrete backing domain for map initialization/metadata and by introducing an optional overlay to visualize sub-domain borders.

Changes:

• Resolve backing (concrete) domains for seamless-domain map init and URL/metadata logic.
• Add a “Seamless Borders” preference + settings UI and draw border/label overlays on the map.
• Update popup value lookup and various typing/import adjustments to accommodate seamless domains.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/routes/+page.svelte	Initializes map center/zoom using a resolved backing domain when a seamless domain is selected.
src/lib/url.ts	Adjusts next-URL computation to accept seamless domains and resolve a concrete domain for interval logic.
src/lib/stores/preferences.ts	Adds showSeamlessBorders to preferences shape.
src/lib/stores/om-protocol-settings.ts	Refactors imports; continues using getNextOmUrls in postReadCallback.
src/lib/prefetch.ts	Adjusts domain typing but currently still assumes a concrete domain grid for range computation.
src/lib/popup.ts	Adds seamless-aware popup value lookup and resolves concrete domain for bounds checks.
src/lib/metadata.ts	Fetches metadata using backing domain value for seamless domains.
src/lib/layers.ts	Implements seamless border overlay source/layers and updates them on URL/layer refresh.
src/lib/constants.ts	Adds default preference showSeamlessBorders.
src/lib/components/time/time-selector.svelte	Adds non-null assertions for interval fields (needs proper seamless-domain handling).
src/lib/components/settings/settings.svelte	Adds the Seamless Borders settings section to the settings panel.
src/lib/components/settings/seamless-border-settings.svelte	New toggle UI to show/hide seamless sub-domain borders.
src/lib/components/selection/variable-selection.svelte	Class string formatting adjustments.
src/lib/clipping.ts	Tightens typing for coordinate collection and removes any usage in geometry handling.
package.json	Bumps @openmeteo/weather-map-layer to a newer GitHub commit.
package-lock.json	Lockfile update for the bumped @openmeteo/weather-map-layer commit/integrity.

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