fix: bind web server to 127.0.0.1 by default and add --host CLI flag (#750)

## Summary

Fixes #736 — OpenChamber listens on `0.0.0.0` (all interfaces) by default, exposing the server to the network without warning. The log output shows `visit: http://127.0.0.1:...` which is misleading.

## Changes

- **Default bind address changed to `127.0.0.1`** — server is only accessible locally unless explicitly configured otherwise
- **New `--host` CLI flag** — `openchamber --host 0.0.0.0 -p 8080` to listen on all interfaces
- **`OPENCHAMBER_HOST` env var** — documented in help text and docker-compose.yml as an alternative to `--host`
- **Docker entrypoint** defaults to `OPENCHAMBER_HOST=0.0.0.0` so container port mapping continues to work
- **Startup logs** show the actual bind address instead of hardcoded `localhost`

### Resolution priority

```
--host flag  >  OPENCHAMBER_HOST env var  >  127.0.0.1 (default)
```

### What doesn't break

- **Desktop app** — already forces `OPENCHAMBER_HOST=127.0.0.1` via Tauri
- **VS Code extension** — doesn't use the web server
- **Docker** — entrypoint sets `OPENCHAMBER_HOST=0.0.0.0`, preserving current behavior
- **Tunnels** — cloudflared connects to `127.0.0.1` origin internally, works regardless of bind address

## Testing

Automated:
- `bun run type-check` / `bun run lint` — pass

Manual (CLI, direct `node` execution):
- Default bind → `127.0.0.1` (verified via `lsof`/netstat)
- `--host 0.0.0.0` → binds all interfaces
- `--host=0.0.0.0` (inline) → works
- `--host` without value → error exit 2
- `OPENCHAMBER_HOST` env var → respected
- `--host` flag overrides env var
- IPv6 `::1` → correct bracketed URL, health check 200
- CLI daemon start/stop → works
- `visit:` URL → correct
- Help text → `--host` in OPTIONS, `OPENCHAMBER_HOST` in ENVIRONMENT
- Browser UI → loads and works
- Tunnel via UI → works
- Desktop app → no regression

Docker (tested on Ubuntu with native Docker):
- SSH key generated successfully
- `OpenChamber server listening on 0.0.0.0:3000`
- Health check 200
- `uid=1000(openchamber)` confirmed

packages/web/server/index.d.ts

@@ -13,6 +13,7 @@ export interface WebUiServerController {
 
 export interface StartWebUiServerOptions {
   port?: number;
+  host?: string;
   attachSignals?: boolean;
   exitOnShutdown?: boolean;
   uiPassword?: string | null;
@@ -27,6 +28,7 @@ export declare function setupProxy(app: Express): void;
 export declare function restartOpenCode(): Promise<void>;
 export declare function parseArgs(argv?: string[]): {
   port: number;
+  host?: string;
   uiPassword: string | null;
   tryCfTunnel: boolean;
   tunnelProvider?: string;

packages/web/server/index.js

@@ -5788,6 +5788,7 @@ function parseArgs(argv = process.argv.slice(2)) {
 
   const options = {
     port: DEFAULT_PORT,
+    host: undefined,
     uiPassword: envPassword,
     tryCfTunnel: envCfTunnel,
     tunnelProvider: envTunnelProvider,
@@ -5826,6 +5827,13 @@ function parseArgs(argv = process.argv.slice(2)) {
       continue;
     }
 
+    if (optionName === 'host') {
+      const { value, nextIndex } = consumeValue(i, inlineValue);
+      i = nextIndex;
+      options.host = typeof value === 'string' && value.trim().length > 0 ? value.trim() : undefined;
+      continue;
+    }
+
     if (optionName === 'ui-password') {
       const { value, nextIndex } = consumeValue(i, inlineValue);
       i = nextIndex;
@@ -7095,6 +7103,7 @@ async function gracefulShutdown(options = {}) {
 
 async function main(options = {}) {
   const port = Number.isFinite(options.port) && options.port >= 0 ? Math.trunc(options.port) : DEFAULT_PORT;
+  const host = typeof options.host === 'string' && options.host.length > 0 ? options.host : undefined;
   const tryCfTunnel = options.tryCfTunnel === true;
   const shouldUseCanonicalTunnelConfig = typeof options.tunnelMode === 'string'
     || typeof options.tunnelProvider === 'string'
@@ -14235,9 +14244,10 @@ async function main(options = {}) {
 
   let activePort = port;
 
-  const bindHost = typeof process.env.OPENCHAMBER_HOST === 'string' && process.env.OPENCHAMBER_HOST.trim().length > 0
-    ? process.env.OPENCHAMBER_HOST.trim()
-    : null;
+  const bindHost = host
+    || (typeof process.env.OPENCHAMBER_HOST === 'string' && process.env.OPENCHAMBER_HOST.trim().length > 0
+      ? process.env.OPENCHAMBER_HOST.trim()
+      : '127.0.0.1');
 
   await new Promise((resolve, reject) => {
     const onError = (error) => {
@@ -14256,9 +14266,12 @@ async function main(options = {}) {
         // ignore
       }
 
-      console.log(`OpenChamber server running on port ${activePort}`);
-      console.log(`Health check: http://localhost:${activePort}/health`);
-      console.log(`Web interface: http://localhost:${activePort}`);
+      const displayHost = (bindHost === '0.0.0.0' || bindHost === '::' || bindHost === '[::]')
+        ? 'localhost'
+        : (bindHost.includes(':') ? `[${bindHost}]` : bindHost);
+      console.log(`OpenChamber server listening on ${bindHost}:${activePort}`);
+      console.log(`Health check: http://${displayHost}:${activePort}/health`);
+      console.log(`Web interface: http://${displayHost}:${activePort}`);
 
       if (startupTunnelRequest) {
         const startupModeLabel = startupTunnelRequest.mode === TUNNEL_MODE_QUICK
@@ -14308,11 +14321,7 @@ async function main(options = {}) {
       resolve();
     };
 
-    if (bindHost) {
-      server.listen(port, bindHost, onListening);
-    } else {
-      server.listen(port, onListening);
-    }
+    server.listen(port, bindHost, onListening);
   });
 
   if (attachSignals && !signalsAttached) {
@@ -14355,6 +14364,7 @@ if (isCliExecution) {
   exitOnShutdown = true;
   main({
     port: cliOptions.port,
+    host: cliOptions.host,
     tryCfTunnel: cliOptions.tryCfTunnel,
     tunnelProvider: cliOptions.tunnelProvider,
     tunnelMode: cliOptions.tunnelMode,