feat(shell): PTY resize via HTTP + frontend xterm.onResize hook

The PTY was hardcoded to 80x24 at start, so full-screen programs
(htop, vim, less, tmux, top) drew themselves for 80x24 regardless
of the actual xterm window. Add a path for the frontend to push the
real cols/rows to the backend, which calls pty.Setsize on the pty
file.

Backend:
- internal/shell/manager.go: Resize(workspaceID, cols, rows) added to
  the Manager interface and implemented on LocalManager. Looks up the
  session, type-asserts sess.Stdin.(*os.File), calls pty.Setsize.
  Validates cols/rows in [1, 10000] and returns CodeBadRequest on
  bad input, CodeNotFound when no session.
- internal/service/shell_service.go: ShellService.Resize wrapper that
  checks workspace existence first.
- internal/api/shell_handler.go: resize handler, 204 on success.
- internal/api/router.go: register POST /workspaces/:id/shell/resize.
- internal/model/shell.go: ShellResizeRequest{Cols, Rows}.
- internal/shell/manager_test.go: 3 new tests (valid resize via
  pty.Getsize round-trip, invalid size, missing session).

Frontend:
- web/src/lib/api/process.ts: useShellResize mutation hook.
- web/src/components/terminal/TerminalPanel.tsx: terminal.onResize
  subscription with 100ms debounce; filters 0x0; only fires when
  workspaceId is set; cleanup clears timeout + disposes listener.

E2E: 'stty size' after POST /shell/resize {cols:200, rows:50} returns
'50 200'. 0x40 → 400, missing workspace → 404, valid → 204.

Conversation: 019f360a-5eba-7c81-94d3-e5d58ad3c026

web/src/components/terminal/TerminalPanel.tsx

@@ -6,6 +6,7 @@ import { Button } from "@/components/ui/button";
 import {
   useShellStart,
   useShellStatus,
+  useShellResize,
   useShellWebSocket,
 } from "@/lib/api/process";
 
@@ -26,6 +27,8 @@ export function TerminalPanel({ workspaceId }: TerminalPanelProps) {
   const { status, send, onData } = useShellWebSocket(workspaceId, shellRunning);
   const startShell = useShellStart();
 
+  const resize = useShellResize();
+
   // Initialize xterm once and keep it alive across WS status changes.
   useEffect(() => {
     const container = containerRef.current;
@@ -33,6 +36,8 @@ export function TerminalPanel({ workspaceId }: TerminalPanelProps) {
 
     let terminal: Terminal | undefined;
     let fitAddon: FitAddon | undefined;
+    let debounceRef: number | undefined;
+    let removeResize: (() => void) | undefined;
 
     try {
       terminal = new Terminal({
@@ -55,6 +60,20 @@ export function TerminalPanel({ workspaceId }: TerminalPanelProps) {
       fitAddon = new FitAddon();
       terminal.loadAddon(fitAddon);
       terminal.open(container);
+
+      removeResize = terminal.onResize(({ cols, rows }) => {
+        if (!workspaceId) return;
+        if (cols <= 0 || rows <= 0) return;
+        if (debounceRef !== undefined) {
+          window.clearTimeout(debounceRef);
+        }
+        debounceRef = window.setTimeout(() => {
+          debounceRef = undefined;
+          if (!workspaceId) return;
+          resize.mutate({ workspaceId, cols, rows });
+        }, 100);
+      }).dispose;
+
       terminalRef.current = terminal;
       fitAddonRef.current = fitAddon;
     } catch {
@@ -91,6 +110,11 @@ export function TerminalPanel({ workspaceId }: TerminalPanelProps) {
       if (rafId !== undefined) {
         cancelAnimationFrame(rafId);
       }
+      if (debounceRef !== undefined) {
+        window.clearTimeout(debounceRef);
+        debounceRef = undefined;
+      }
+      removeResize?.();
       resizeObserver.disconnect();
       window.removeEventListener("resize", handleResize);
       terminal?.dispose();
@@ -98,7 +122,7 @@ export function TerminalPanel({ workspaceId }: TerminalPanelProps) {
       terminalRef.current = null;
       fitAddonRef.current = null;
     };
-  }, []);
+  }, [resize, workspaceId]);
 
   // Reset per-workspace state and wire the active socket to xterm.
   useEffect(() => {

web/src/lib/api/process.ts

@@ -148,7 +148,31 @@ export function useShellStop(): UseMutationResult<void, Error, string> {
 }
 
 export function useShellRestart(): UseMutationResult<void, Error, string> {
-  return useShellActionMutation("restart");
+	return useShellActionMutation("restart");
+}
+
+export interface ShellResizeVariables {
+	workspaceId: string;
+	cols: number;
+	rows: number;
+}
+
+export function useShellResize(): UseMutationResult<void, Error, ShellResizeVariables> {
+	return useMutation({
+		mutationFn: async ({ workspaceId, cols, rows }) => {
+			const res = await apiClient(
+				`/api/workspaces/${encodeURIComponent(workspaceId)}/shell/resize`,
+				{
+					method: "POST",
+					headers: { "Content-Type": "application/json" },
+					body: JSON.stringify({ cols, rows }),
+				},
+			);
+			if (!res.ok) {
+				throw new Error(`Failed to resize shell: ${res.status}`);
+			}
+		},
+	});
 }
 
 export type ProcessWebSocketStatus =