RBAC: Enforce read-only viewer role across backend and frontend

- Add global WriteAccessGuard that blocks POST/PUT/PATCH/DELETE for viewer role
- Add @AllowViewer() decorator for endpoints viewers need (switch-org, intro-seen, AI recommendations)
- Add useIsReadOnly hook to auth store for frontend role checks
- Hide write UI (add/edit/delete/import buttons, inline editors) in all 13 data pages for viewers
- Disable inline NumberInputs on Budgets and Monthly Actuals pages for viewers
- Skip onboarding wizard for viewer role users

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

backend/src/common/decorators/allow-viewer.decorator.ts

@@ -0,0 +1,4 @@
+import { SetMetadata } from '@nestjs/common';
+
+export const ALLOW_VIEWER_KEY = 'allowViewer';
+export const AllowViewer = () => SetMetadata(ALLOW_VIEWER_KEY, true);

backend/src/common/guards/write-access.guard.ts

@@ -0,0 +1,34 @@
+import { Injectable, CanActivate, ExecutionContext, ForbiddenException } from '@nestjs/common';
+import { Reflector } from '@nestjs/core';
+import { ALLOW_VIEWER_KEY } from '../decorators/allow-viewer.decorator';
+
+@Injectable()
+export class WriteAccessGuard implements CanActivate {
+  constructor(private reflector: Reflector) {}
+
+  canActivate(context: ExecutionContext): boolean {
+    const request = context.switchToHttp().getRequest();
+    const method = request.method;
+
+    // Allow all read methods
+    if (['GET', 'HEAD', 'OPTIONS'].includes(method)) return true;
+
+    // If no user on request (unauthenticated endpoints like login/register), allow
+    const user = request.user;
+    if (!user) return true;
+
+    // Check for @AllowViewer() exemption on handler or class
+    const allowViewer = this.reflector.getAllAndOverride<boolean>(ALLOW_VIEWER_KEY, [
+      context.getHandler(),
+      context.getClass(),
+    ]);
+    if (allowViewer) return true;
+
+    // Block viewer role from write operations
+    if (user.role === 'viewer') {
+      throw new ForbiddenException('Read-only users cannot modify data');
+    }
+
+    return true;
+  }
+}