fix: SPA fallback via HttpExceptionFilter for non-API 404s

Move SPA fallback from main.ts catch-all (didn't work - NestJS 404
handler runs first) to HttpExceptionFilter. Now non-API GET 404s
serve index.html so client-side routing works for /dashboard/* etc.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/common/filters/http-exception.filter.ts

@@ -1,13 +1,16 @@
 import { ExceptionFilter, Catch, ArgumentsHost, HttpException, HttpStatus, Logger } from "@nestjs/common";
-import { Response } from "express";
+import { Request, Response } from "express";
+import { join } from "path";
 import { trace, SpanStatusCode } from "@opentelemetry/api";
 
 @Catch()
 export class HttpExceptionFilter implements ExceptionFilter {
   private readonly logger = new Logger("ExceptionFilter");
+  private readonly indexPath = join(__dirname, "..", "..", "..", "..", "web", "dist", "index.html");
 
   catch(exception: unknown, host: ArgumentsHost) {
     const ctx = host.switchToHttp();
+    const request = ctx.getRequest<Request>();
     const response = ctx.getResponse<Response>();
 
     let status = HttpStatus.INTERNAL_SERVER_ERROR;
@@ -30,6 +33,11 @@ export class HttpExceptionFilter implements ExceptionFilter {
       this.logger.error(`Unhandled error: ${exception.message}`, exception.stack);
     }
 
+    // SPA fallback: serve index.html for non-API GET 404s
+    if (status === HttpStatus.NOT_FOUND && request.method === "GET" && !request.path.startsWith("/api")) {
+      return response.sendFile(this.indexPath);
+    }
+
     // Record error on active OTel span
     const span = trace.getActiveSpan();
     if (span) {