feat(03-01): add searchMessages backend with LIKE query, IPC channel, preload bridge, and tests

- Add searchMessages method to AppDatabase with LIKE query, wildcard escaping, channel scoping, ORDER BY created_at DESC with limit
- Add searchMessages IPC channel and searchMessagesSchema to contracts.ts
- Register IPC handler in ipc-agent-coordination-handlers.ts with Zod validation
- Wire searchMessages through preload bridge
- Add 8 unit tests covering body/subject search, wildcard escaping, channel scoping, empty results, limit, and ordering

src/preload/index.ts

@@ -70,6 +70,7 @@ const api: DesktopApi = {
   getAutomationLog: (companyId) => ipcRenderer.invoke(IPC_CHANNELS.getAutomationLog, companyId),
   sendAgentMessage: (input) => ipcRenderer.invoke(IPC_CHANNELS.sendAgentMessage, input),
   listAgentMessages: (input) => ipcRenderer.invoke(IPC_CHANNELS.listAgentMessages, input),
+  searchMessages: (input) => ipcRenderer.invoke(IPC_CHANNELS.searchMessages, input),
   markMessageRead: (input) => ipcRenderer.invoke(IPC_CHANNELS.markMessageRead, input),
   saveWorkflow: (input) => ipcRenderer.invoke(IPC_CHANNELS.saveWorkflow, input),
   deleteWorkflow: (input) => ipcRenderer.invoke(IPC_CHANNELS.deleteWorkflow, input),