feat: support deploying pi-web under an arbitrary base path

Make all browser-facing API paths and WebSocket URLs relative so that
vite's <base href> can resolve them correctly when pi-web is served
from a subpath (e.g. /ai or /test/ai).

Also introduce PI_WEB_BASE_PATH so the server can generate plugin
module URLs that include the base path.

src/server/piWebPluginService.ts

@@ -50,6 +50,11 @@ interface PiWebPluginServiceOptions {
   configProvider?: () => PiWebConfig;
 }
 
+function piWebBasePath(): string {
+  const basePath = process.env["PI_WEB_BASE_PATH"] ?? "";
+  return basePath.replace(/\/$/u, "");
+}
+
 interface LocalPluginRoot {
   path: string;
   source: string;
@@ -135,7 +140,7 @@ export class PiWebPluginService {
   private pluginInfo(plugin: PluginRecord, config: PiWebConfig): PiWebPluginInfo {
     return {
       id: plugin.id,
-      module: `/pi-web-plugins/${encodeURIComponent(plugin.id)}/${plugin.entryFile}?${pluginModuleQuery(plugin)}`,
+      module: `${piWebBasePath()}/pi-web-plugins/${encodeURIComponent(plugin.id)}/${plugin.entryFile}?${pluginModuleQuery(plugin)}`,
       source: plugin.source,
       scope: plugin.scope,
       machineSpecific: plugin.machineSpecific,