Fix test isolation to prevent loading real user config

Tests were picking up real ~/.warelay/warelay.json with emojis and
prefixes (like "🦞"), causing test assertions to fail. Added proper
config mocks to all test files.

Changes:
- Mock loadConfig() in index.core.test.ts, inbound.media.test.ts,
  monitor-inbox.test.ts
- Update test-helpers.ts default mock to disable all prefixes
- Tests now use clean config: no messagePrefix, no responsePrefix,
  no timestamp, allowFrom=["*"]

This ensures tests validate core behavior without user-specific config.
The responsePrefix feature itself is already fully config-driven - this
only fixes test isolation.

src/web/inbound.media.test.ts

@@ -9,6 +9,9 @@ vi.mock("../config/config.js", () => ({
   loadConfig: vi.fn().mockReturnValue({
     inbound: {
       allowFrom: ["*"], // Allow all in tests
+      messagePrefix: undefined,
+      responsePrefix: undefined,
+      timestampPrefix: false,
     },
   }),
 }));

src/web/monitor-inbox.test.ts

@@ -13,6 +13,9 @@ vi.mock("../config/config.js", () => ({
   loadConfig: vi.fn().mockReturnValue({
     inbound: {
       allowFrom: ["*"], // Allow all in tests
+      messagePrefix: undefined,
+      responsePrefix: undefined,
+      timestampPrefix: false,
     },
   }),
 }));

src/web/test-helpers.ts

@@ -3,14 +3,28 @@ import { vi } from "vitest";
 import type { MockBaileysSocket } from "../../test/mocks/baileys.js";
 import { createMockBaileys } from "../../test/mocks/baileys.js";
 
-let loadConfigMock: () => unknown = () => ({});
+let loadConfigMock: () => unknown = () => ({
+  inbound: {
+    allowFrom: ["*"], // Allow all in tests by default
+    messagePrefix: undefined, // No message prefix in tests
+    responsePrefix: undefined, // No response prefix in tests
+    timestampPrefix: false, // No timestamp in tests
+  },
+});
 
 export function setLoadConfigMock(fn: () => unknown) {
   loadConfigMock = fn;
 }
 
 export function resetLoadConfigMock() {
-  loadConfigMock = () => ({});
+  loadConfigMock = () => ({
+    inbound: {
+      allowFrom: ["*"],
+      messagePrefix: undefined,
+      responsePrefix: undefined,
+      timestampPrefix: false,
+    },
+  });
 }
 
 vi.mock("../config/config.js", () => ({