Add native Zen Browser support

[agats]

Summary

Adds native (non-Flatpak) Zen Browser support as a target browser.

Zen support currently only covers the Flatpak build (BROWSER_TYPE_ZEN_FLATPAK, added in #336). Users who install Zen natively — e.g. via AUR's zen-browser-bin on Arch, or Zen's own installer/tarball — have no matching entry, even though os.path.exists(browser.test_path) is the only gate the GUI applies.

Zen is a Firefox fork and behaves identically to the other native Firefox-family entries already in this list (LibreWolf, Waterfox, Floorp) for the purposes of webapp-manager's profile handling, so this reuses the existing BROWSER_TYPE_FIREFOX type rather than adding a new one — each webapp gets its own isolated profile the same way native Firefox/LibreWolf/Waterfox/Floorp webapps already do, with zero new branching logic required.

Browser(BROWSER_TYPE_FIREFOX, "Zen Browser", "zen-browser", "/usr/bin/zen-browser"),

The welcome-wizard problem, and how this avoids it

A fresh, empty Firefox-family profile normally triggers Zen's first-run welcome/setup screen. Left unaddressed, that would pop up on every newly created Zen webapp — see the discussion on #335, where a workaround gist (linked below) works around this by sharing one profile across all Zen-native webapps, sacrificing the per-app isolation that's the whole point of this being a "web app" in the first place, and requiring manual one-time setup (create a profile via about:profiles, hand-edit a hardcoded path into common.py).

This PR fixes the actual cause instead of working around it: setting zen.welcome-screen.seen=true in the shared profile template (usr/share/webapp-manager/firefox/profile/user.js) suppresses the wizard directly. This pref is Zen-specific and silently ignored by every other Firefox-family browser that shares this same template, so it's a no-op for Firefox/LibreWolf/Waterfox/Floorp. Net effect: full per-app profile isolation, preserved, with no manual setup required and no wizard interruption.

Related

• Closes/relates to Add Zen browser support #335 ("Add Zen browser support" — Flatpak was covered by Add Support for Zen Browser (#335) #336, this covers native).
• Differs from the community workaround gist linked in Add Zen browser support #335 (shared profile + manual setup) by keeping isolation and requiring no setup.
• May also help with Zen browser Webapps open without a seperate taskbar icon #352 (Zen webapps not getting separate taskbar icons) for users who can use the native build instead of Flatpak — the native entry here uses --class WebApp-<codename> the same as every other native Firefox-family entry, and I have not observed the icon-grouping issue with it.

Testing

Verified locally on Arch Linux (webapp-manager 1.4.6) with zen-browser-bin (native, non-Flatpak Zen install):

• "Zen Browser" appears in the browser dropdown when creating/editing a webapp.
• Created and used several webapps against it; each gets its own isolated profile under ~/.local/share/ice/firefox/<codename>/, no shared state between apps.
• No welcome-wizard interruption on any newly created profile after the user.js change.
• Confirmed the two changes here don't affect any other browser type (both are strictly additive / no-op for non-Zen browsers).