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>

src/shared/domain/terminal.ts

@@ -0,0 +1,12 @@
+export interface TerminalSnapshot {
+  cwd: string;
+  shell: string;
+  pid: number;
+  cols: number;
+  rows: number;
+}
+
+export interface TerminalExitInfo {
+  exitCode: number;
+  signal?: number;
+}

src/shared/domain/tooling.ts

@@ -63,6 +63,7 @@ export interface WorkspaceSettings {
   theme: AppearanceTheme;
   tooling: WorkspaceToolingSettings;
   discoveredUserTooling: DiscoveredToolingState;
+  terminalHeight?: number;
 }
 
 export interface SessionToolingSelection {
@@ -185,7 +186,17 @@ export function normalizeTheme(value?: string): AppearanceTheme {
   return validThemes.has(value ?? '') ? (value as AppearanceTheme) : 'dark';
 }
 
+export function normalizeTerminalHeight(value?: number): number | undefined {
+  if (typeof value !== 'number' || !Number.isFinite(value)) {
+    return undefined;
+  }
+
+  const normalized = Math.round(value);
+  return normalized >= 120 ? normalized : undefined;
+}
+
 export function normalizeWorkspaceSettings(settings?: Partial<WorkspaceSettings>): WorkspaceSettings {
+  const terminalHeight = normalizeTerminalHeight(settings?.terminalHeight);
   return {
     theme: normalizeTheme(settings?.theme),
     tooling: {
@@ -193,6 +204,7 @@ export function normalizeWorkspaceSettings(settings?: Partial<WorkspaceSettings>
       lspProfiles: (settings?.tooling?.lspProfiles ?? []).map(normalizeLspProfileDefinition),
     },
     discoveredUserTooling: normalizeDiscoveredToolingState(settings?.discoveredUserTooling),
+    ...(terminalHeight !== undefined ? { terminalHeight } : {}),
   };
 }