M2: frontend walkthrough fixes + explicit dev compose stack

Post-M2 self-walkthrough polish, batched into one commit.

Map / heat:
- fix heat-layer white-screen crash after login (add layer to map before
  setLatLngs; an off-map leaflet.heat layer has a null _map and throws)
- normalize each heat layer to the densest pixel cell visible in the CURRENT
  viewport (maxZoom:0 so intensity factor f=1) and recompute on moveend/zoomend,
  so sparse poo data reaches red and stays normalized at any zoom level
- dark CARTO basemap tiles when the color scheme is dark

UI:
- dark-mode toggle in the top-right, beside the settings gear
- switch top-right nav (records / theme / settings / logout) to Feather icons
  with hover tooltips
- home: Grafana-style quick time-range presets + back/forward shift buttons,
  placed between the From/To pickers and Apply; fix Select/tooltip z-index
  (Leaflet stacking) and the shift-button height alignment

API client:
- stop flooding GET /api/session with 401s: the session probe and the login
  endpoint own their 401s (no global redirect), which fixes the logout hang and
  the spinning login page

Compose:
- rename docker-compose.override.yml -> docker-compose.dev.yml as an explicit,
  non-auto-layered dev stack (8001, -dev container names, prod-copy ./data DB);
  update tests/test_deployment.py (read dev.yml, tolerate the !override tag) and
  the README "Docker Compose" section

Tests:
- pixel-grid peak counter, time-range presets, heat-layer ordering regression,
  and 401-redirect regression

frontend/package-lock.json

@@ -19,6 +19,7 @@
         "openapi-fetch": "^0.17.0",
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
+        "react-feather": "^2.0.10",
         "react-leaflet": "^4.2.1",
         "react-router-dom": "^6.30.4"
       },
@@ -5197,7 +5198,6 @@
       "version": "4.1.1",
       "resolved": "https://registry.npmjs.org/object-assign/-/object-assign-4.1.1.tgz",
       "integrity": "sha512-rJgTQnkUnH1sFw8yT6VSU3zD3sWmu6sZhIseY8VX+GRu3P6F7Fu+JNDoXfklElbLJSnc3FUQHVe4cU5hj+BcUg==",
-      "dev": true,
       "license": "MIT",
       "engines": {
         "node": ">=0.10.0"
@@ -5638,7 +5638,6 @@
       "version": "15.8.1",
       "resolved": "https://registry.npmjs.org/prop-types/-/prop-types-15.8.1.tgz",
       "integrity": "sha512-oj87CgZICdulUohogVAR7AjlC0327U4el4L6eAvOqCeudMDVU0NThNaV+b9Df4dXgSP1gXMTnPdhfe/2qDH5cg==",
-      "dev": true,
       "license": "MIT",
       "dependencies": {
         "loose-envify": "^1.4.0",
@@ -5650,7 +5649,6 @@
       "version": "16.13.1",
       "resolved": "https://registry.npmjs.org/react-is/-/react-is-16.13.1.tgz",
       "integrity": "sha512-24e6ynE2H+OKt4kqsOvNd8kBpV65zoxbA4BVsEOB3ARVWQki/DHzaUoC5KuON/BiccDaCCTZBuOcfZs70kR8bQ==",
-      "dev": true,
       "license": "MIT"
     },
     "node_modules/punycode": {
@@ -5688,6 +5686,18 @@
         "react": "^18.3.1"
       }
     },
+    "node_modules/react-feather": {
+      "version": "2.0.10",
+      "resolved": "https://registry.npmjs.org/react-feather/-/react-feather-2.0.10.tgz",
+      "integrity": "sha512-BLhukwJ+Z92Nmdcs+EMw6dy1Z/VLiJTzEQACDUEnWMClhYnFykJCGWQx+NmwP/qQHGX/5CzQ+TGi8ofg2+HzVQ==",
+      "license": "MIT",
+      "dependencies": {
+        "prop-types": "^15.7.2"
+      },
+      "peerDependencies": {
+        "react": ">=16.8.6"
+      }
+    },
     "node_modules/react-is": {
       "version": "17.0.2",
       "resolved": "https://registry.npmjs.org/react-is/-/react-is-17.0.2.tgz",