fix(setDockIcon): use setActivationPolicy('accessory') for reliable Dock-hiding on Tahoe + previous macOS

[Austin519]

Summary

The current setDockIcon() implementation calls app.dock.hide() followed by app.show() when hide_dock_icon=true. This is unreliable in several ways, and on macOS 26 (Tahoe) it does not work at all.

This PR replaces both calls with app.setActivationPolicy('accessory'|'regular') — the proper Electron API for "background-only app, no Dock presence, no app-switcher presence." It is the runtime equivalent of LSUIElement=true in Info.plist.

Why the current implementation breaks

1. app.show() re-promotes to Dock. app.show() focuses the application, which on macOS has the side effect of re-activating the Dock icon. The very next line after app.dock.hide() undoes the hide.

2. app.dock.hide() doesn't survive BrowserWindow creation. Even when called before any window exists, subsequent window-creation calls re-promote the app to the Dock. The secondary setDockIcon() call in initServerComponents() doesn't reliably catch this.

3. On macOS 26 (Tahoe), app.dock.hide() is a no-op. Verified 2026-05-22 on Tahoe 26.4.1 build 25E253 with BB v1.9.9 + Electron's bundled version. The Dock icon stays visible:

   • From startup with hide_dock_icon=1 in config — Dock icon stays
   • From in-app UI toggle of "Hide Dock Icon" while BB is running — Dock icon stays

Fix

if (hideDockIcon) {
    app.setActivationPolicy("accessory");
} else {
    app.setActivationPolicy("regular");
}

setActivationPolicy('accessory') tells macOS at the AppKit layer that this is an accessory app — no Dock, no app-switcher entry, no menu bar. It survives window creation cycles and works on every macOS version that supports the API (10.6+).

setActivationPolicy('regular') restores the default — the equivalent of the previous app.dock.show() path.

Compatibility

• No behavior change for users with hide_dock_icon=false (the default).
• Users who had hide_dock_icon=true were silently getting no effect on Tahoe; this fixes that.
• Works on macOS 10.6+ (the API is older than the minimum supported macOS). No new dependencies.

Test plan

• macOS 26.4.1 (Tahoe), Apple Silicon (M2 Mac mini), BB v1.9.9 — verified app.dock.hide() doesn't work
• macOS 14 (Sonoma) — would appreciate maintainer verification
• macOS 13 (Ventura) — would appreciate maintainer verification

Happy to iterate or split into smaller commits if preferred.