[BUG] Clicking dock icon spawns new Claude desktop instance instead of focusing existing window (macOS)

Summary

On macOS, clicking the Claude icon in the dock to return to a backgrounded Claude desktop window spawns a new app instance/window instead of bringing the existing window to the front. Repeating this each time I switch away from Claude leaves a long row of stale duplicate windows that I can't easily get back to or close.

This contradicts standard macOS app behavior (every other app focuses its existing window on dock click) and conflicts with the single-window-per-instance architecture documented in #30154.

Repro

1. Open Claude Code desktop, start or resume a session
2. Click any other app (browser, Finder, terminal) to switch focus
3. Click the Claude icon in the dock to return

Expected: The existing Claude window comes to front, with the session I was just in.

Actual: A new Claude window opens (fresh sidebar state, no active session selected). The original window stays in the background and is not surfaced. Repeating steps 2–3 stacks more orphaned windows.

Impact

• Can't multitask between Claude desktop and any other app without piling up duplicate windows
• Forces Cmd+Tab or Mission Control as a workaround (most users will reach for the dock icon by reflex)
• Easy to lose track of which window has the active session — orphaned windows look identical to the active one
• Pile-up only resolves with Quit/Force-Quit + relaunch

Workarounds I've tried

• Cmd+Tab back to Claude — works, focuses existing window
• Mission Control — shows all the orphaned windows, can click into the right one
• Quit Claude and relaunch — clears the pile but loses any in-flight work

Environment

• macOS 26.5 (build 25F71), Apple Silicon (M4 MacBook Air)
• Claude Desktop 1.8555.2
• Claude Code CLI 2.1.45
• Plan: Max

Here's a redacted screenshot: <img width="2000" height="1183" alt="Image" src="https://github.com/user-attachments/assets/37b959da-089b-4356-bcc8-a3454c72100b" />