Add contour lines to the Overlays panel via maplibre-contour

5th Overlays item, generated client-side from the same online Terrarium
DEM tiles Terrain shading already fetches (no offline fallback — the
plugin's DemSource has no public hook to route through the pmtiles://
protocol the offline basemap uses, so this quietly renders nothing
without a network, same as Radar/the Wikipedia layer).

A plain vector source/layer under MapLibre's own tile lifecycle, no
manual refetch-on-move code needed. Thresholds are the plugin's own
generic starting point, not yet tuned against real terrain. Labels
skipped for now (font-name matching across the online/offline styles'
own glyph sets adds real risk for a nice-to-have, not the actual ask).

public/vendor/maplibre-contour/maplibre-contour.min.js

@@ -0,0 +1,15 @@
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+/*! *****************************************************************************
+    Copyright (c) Microsoft Corporation.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose with or without fee is hereby granted.
+
+    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+    REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+    AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+    INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+    LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+    OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+    PERFORMANCE OF THIS SOFTWARE.
+    ***************************************************************************** */function o(t,e){var i={};for(var r in t)Object.prototype.hasOwnProperty.call(t,r)&&e.indexOf(r)<0&&(i[r]=t[r]);if(null!=t&&"function"==typeof Object.getOwnPropertySymbols){var s=0;for(r=Object.getOwnPropertySymbols(t);s<r.length;s++)e.indexOf(r[s])<0&&Object.prototype.propertyIsEnumerable.call(t,r[s])&&(i[r[s]]=t[r[s]])}return i}function a(t,e,i,r){return new(i||(i=Promise))((function(s,n){function o(t){try{h(r.next(t))}catch(t){n(t)}}function a(t){try{h(r.throw(t))}catch(t){n(t)}}function h(t){var e;t.done?s(t.value):(e=t.value,e instanceof i?e:new i((function(t){t(e)}))).then(o,a)}h((r=r.apply(t,e||[])).next())}))}function h(t){const e=Object.entries(t);return e.sort((([t],[e])=>t<e?-1:t>e?1:0)),e}function l(t){return h(t).map((([t,e])=>[t,..."number"==typeof e?[e]:e].join("*"))).join("~")}function c(t){return h(t).map((([t,e])=>`${encodeURIComponent(t)}=${encodeURIComponent(e)}`)).join(",")}let 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