feat: move base URL, API key, and model to server .env

- BASE_URL, API_KEY, MODEL now read from process.env (Bun auto-loads .env)
- requireEnv() fails fast at startup if any is missing
- request body simplifies to { prompt, size, referenceImages? }
- client drops the three fields from form and localStorage
- add .env.example as the variable-name source of truth
- AGENTS.md notes the 0.0.0.0 bind now exposes the upstream quota to
  anyone reachable on the network

AGENTS.md

@@ -12,6 +12,26 @@ proxy-read timeout.
   `dotenv` — Bun loads `.env` automatically.
 - Bun version baseline: `1.3.13` (per `README.md`).
 
+## Config
+
+Required env vars (validated at startup via `requireEnv`; the process exits
+if any is missing):
+
+| Var | Example | Purpose |
+|---|---|---|
+| `BASE_URL` | `https://api.openai.com/v1` | OpenAI-compatible base URL |
+| `API_KEY` | `sk-…` | Bearer token sent to upstream |
+| `MODEL` | `gpt-image-2` | Model name forwarded to upstream |
+
+`.env.example` is the source of truth for variable names. The real `.env`
+is gitignored. Restart the server after changing env vars — they are read
+once at module load.
+
+These secrets stay **server-side**. The browser only sends `prompt`,
+`size`, and `referenceImages`. Combined with the `0.0.0.0` bind, this
+means anyone reachable on the network can spend your upstream quota —
+bind to `127.0.0.1` or put auth in front if that matters.
+
 ## Commands
 
 - Install: `bun install`
@@ -41,7 +61,9 @@ Three files do everything:
     connections before the first keepalive can fire. The symptom is an
     empty EventStream in DevTools and `request timed out after 10 seconds`
     in the log.
-  - `POST /api/generate` uses `streamSSE` from `hono/streaming`. Emits:
+  - `POST /api/generate` uses `streamSSE` from `hono/streaming`. Accepts
+    `{ prompt, size, referenceImages? }` — `BASE_URL`, `API_KEY`, and
+    `MODEL` come from env, not the request. Emits:
     - `event: partial` — `{ image: dataUrl, index }` for each
       `image_generation.partial_image` / `image_edit.partial_image`.
     - `event: final` — `{ image: dataUrl }` for `*.completed`.
@@ -80,9 +102,8 @@ Three files do everything:
     `signal`, and the `onopen` / `onmessage` / `onerror` callbacks.
   - On `done`, the client calls `abort.abort()` to terminate the
     `fetchEventSource` loop cleanly — otherwise it would retry forever.
-  - Text fields (`baseURL`, `apiKey`, `model`, `size`, `prompt`) persist
-    in `localStorage` under the `aip:<field>` prefix. Reference images
-    stay in-memory only.
+  - Text fields (`size`, `prompt`) persist in `localStorage` under the
+    `aip:<field>` prefix. Reference images stay in-memory only.
 - `index.html` — markup + inline CSS only. No JS lives here.
 
 No router, no DB, no auth, no AI SDK. API key is supplied per-request by the