feat: add integrated terminal backend

- add a PTY manager with platform shell resolution and streamed data/exit events
- expose terminal lifecycle and terminal height IPC through preload and app service
- persist terminal panel height in workspace settings and document the backend contract
- add backend tests and validate native packaging with bun run package

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/shared/tooling.test.ts

@@ -21,6 +21,7 @@ const TIMESTAMP = '2026-03-23T00:00:00.000Z';
 describe('tooling settings helpers', () => {
   test('normalizes persisted MCP and LSP definitions into trimmed stable settings', () => {
     const workspaceSettings = normalizeWorkspaceSettings({
+      terminalHeight: 240.4,
       tooling: {
         mcpServers: [
           {
@@ -64,6 +65,7 @@ describe('tooling settings helpers', () => {
 
     expect(workspaceSettings).toEqual({
       theme: 'dark',
+      terminalHeight: 240,
       tooling: {
         mcpServers: [
           {
@@ -109,6 +111,11 @@ describe('tooling settings helpers', () => {
     });
   });
 
+  test('drops invalid persisted terminal height values', () => {
+    expect(normalizeWorkspaceSettings({ terminalHeight: 119 }).terminalHeight).toBeUndefined();
+    expect(normalizeWorkspaceSettings({ terminalHeight: Number.NaN }).terminalHeight).toBeUndefined();
+  });
+
   test('validates required MCP transport settings', () => {
     const localServer: McpServerDefinition = {
       id: 'mcp-local',