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.

views/layout.erb

@@ -30,11 +30,16 @@
   <script type="importmap">
     {
       "imports": {
+        "/js/auth.js": "/js/auth.js?v=<%= ASSET_VERSION %>",
         "/js/gnss.js": "/js/gnss.js?v=<%= ASSET_VERSION %>",
         "/js/gpx.js": "/js/gpx.js?v=<%= ASSET_VERSION %>",
         "/js/icons.js": "/js/icons.js?v=<%= ASSET_VERSION %>",
         "/js/livetrack.js": "/js/livetrack.js?v=<%= ASSET_VERSION %>",
+        "/js/locate.js": "/js/locate.js?v=<%= ASSET_VERSION %>",
+        "/js/measure.js": "/js/measure.js?v=<%= ASSET_VERSION %>",
+        "/js/notes.js": "/js/notes.js?v=<%= ASSET_VERSION %>",
         "/js/panels.js": "/js/panels.js?v=<%= ASSET_VERSION %>",
+        "/js/plugins.js": "/js/plugins.js?v=<%= ASSET_VERSION %>",
         "/js/radar.js": "/js/radar.js?v=<%= ASSET_VERSION %>",
         "/js/track_profile.js": "/js/track_profile.js?v=<%= ASSET_VERSION %>"
       }