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

README.md

@@ -338,18 +338,20 @@ python scripts/export_openapi.py
 
 当前 Compose 分成两层：
 
-- `docker-compose.yml`：默认使用 registry image，适合部署 / 生产拉取
-- `docker-compose.override.yml`：仅为本地开发追加 `build: .`
+- `docker-compose.yml`：默认使用 registry image，适合部署 / 生产拉取（暴露 8881）
+- `docker-compose.dev.yml`：本地开发显式叠加层——追加 `build: .`、独立 project /
+  容器名（`-dev` 后缀）、暴露 8001，并把 DB 指向挂载的 `./data` 副本，可与生产栈在同一台机器上并存
 
-本地开发启动方式：
+本地开发启动方式（显式叠加 dev 层）：
 
 ```bash
-docker compose up -d --build
+docker compose -f docker-compose.yml -f docker-compose.dev.yml up -d --build
 ```
 
-上面的命令会自动叠加 `docker-compose.override.yml`，因此本地仍然会按当前工作目录重新 build。
+dev 层刻意不沿用 `docker-compose.override.yml` 这种会被 `docker compose up` 自动叠加的文件名，
+因此默认的 `docker compose up` 只用生产基础文件，不会把开发端口 / 配置误带到生产。
 
-如果要按生产方式直接从 registry 拉取并启动，显式只使用基础 compose 文件：
+如果要按生产方式直接从 registry 拉取并启动，使用基础 compose 文件：
 
 ```bash
 docker compose -f docker-compose.yml pull