Stop the flow store leaking git zombies, honour VITE_API_URL from .env

`FlowStore._git` now passes `-c gc.auto=0`, so `git commit` no longer forks a
background `gc --auto` that reparents onto PID 1 and stays there unreaped. With
auto-gc off nothing packs on its own, so `_commit` runs a foreground `git gc`
every 500 commits — the trigger a long-lived seeding session actually reaches.

`docker/compose.yml` takes the frontend's `VITE_API_URL` build arg from the
environment, keeping `https://api.${DOMAIN}` only as the fallback. `setup.sh`
already derives the scheme from `ENVIRONMENT`, so an `up --build` that does not
layer `compose.local.yml` stops shipping a bundle that calls `https://api.localhost`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01StpRc2C6au1WJ1EUU7fsfu

.env.example

@@ -11,6 +11,11 @@ FRONTEND_HOST=http://localhost:5173
 # local, staging, production
 ENVIRONMENT=local
 
+# Baked into the SPA at build time: the address the interface calls. The
+# workspace root's scripts/setup.sh derives it from DOMAIN and ENVIRONMENT —
+# plain http for a local target, where nothing terminates TLS.
+VITE_API_URL=http://api.localhost
+
 PROJECT_NAME="Fluksio"
 STACK_NAME=fluksio-app