Add an import plugin system, with the Overpass POI query as its first plugin

A plugin is a directory under plugins/ with a plugin.json manifest
({"label": "...", "command": "./run.sh"}) plus a script that reads
--south/--west/--north/--east and prints a GeoJSON FeatureCollection
to stdout (lib/maptool/plugins.rb) — that's the entire interface a
plugin needs to know about this app. The sidebar's new import icon
(logged in only, GET /api/plugins + POST /api/plugins/:name/run) lists
whatever's found and runs one for the current map viewport, saving its
output as a new .geojson file the same way an upload would be.

The point: a scraper for a site whose ToS forbids that (Park4Night,
iOverlander, Freecampsites) doesn't need to live in this public repo
to work here — its own directory just goes under plugins/ locally and
is never git add'ed, no special-casing needed anywhere.

plugins/overpass-pois wraps the existing rake overpass:pois query as
the first, in-repo, reference example of the contract.

Also fills in four modules (auth.js, locate.js, measure.js, notes.js)
missing from layout.erb's cache-busting importmap alongside adding
plugins.js's own entry — a pre-existing gap that meant they could keep
serving a stale cached copy across a deploy despite the importmap
existing specifically to prevent that.

public/css/style.css

@@ -1439,3 +1439,60 @@ html, body {
 .profile-panel__tooltip-label {
   color: #c3c2b7;
 }
+
+.plugins-panel__hint {
+  margin: 0 0 6px;
+  max-width: 200px;
+  color: #888;
+  font-size: 11px;
+}
+
+.plugins-panel__list {
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+
+.plugins-panel__item {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 4px 0;
+  white-space: nowrap;
+}
+
+.plugins-panel__label {
+  flex: 1;
+}
+
+.plugins-panel__run {
+  /* See .notes-panel__actions button's own comment — without this some
+     browsers paint their own hover/active highlight over a custom
+     background. */
+  appearance: none;
+  padding: 4px 10px;
+  border: none;
+  border-radius: 4px;
+  background: #1a5fb4;
+  color: #fff;
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+}
+
+.plugins-panel__run:disabled {
+  opacity: 0.6;
+  cursor: default;
+}
+
+.plugins-panel__status {
+  min-width: 3.5em;
+  color: #888;
+  font-size: 11px;
+}
+
+.plugins-panel__empty {
+  color: #888;
+  white-space: normal;
+  max-width: 200px;
+}