fix: mixed content, external links, --help, and crash recovery

[pol4rfuchs]

• allowRunningInsecureContent: true — fixes subscribing to self-hosted http:// instances from the https:// ntfy.sh web UI (mixed content was silently blocked by Chromium, button click registered but request never fired) (cannot subscribe topic from internal server #139)
• will-navigate handler — links clicked inside the app now open in the system browser instead of navigating the main window away (🐛 Bug: Clicking links opens them inside the ntfy-desktop application #155)
• --help/-h now prints usage and exits before any window is created, instead of silently starting the app (Bug: --help and -h just start the program without any indication the program supports CLI args #158)
• render-process-gone handler — reload instead of a dead/white window if the loaded ntfy.sh or self-hosted web UI crashes on its own (crash when clicking on subscribe topic #162, root cause is upstream frontend code, this only recovers from it)

Pull Request

Select which topic best describes your contribution:

• Feature
• Bug
• Documentation / Wiki

---

Description

Explain here what your pull request includes and what you've done

---

Before You Submit

Please ensure you check the following items to indicate that you've read this section and completed each task

• My code follows the Contribution Guidelines
• I give expressed consent for my work to be used in this repo
• I have tested my work and it functions as intended
• I have included documentation if the change requires such