Launchers: apps' own desktop entries, rewritten; keyring moves into them

steamFrame.launchers.<desktop ID> takes the app's own entry (a Flatpak's
export, a Nix package's or a host file) and rewrites only its command lines
(hostEnv, env, flatpakArgs, args, wrappers) and the keys asked for
(mimeTypes, defaultFor, settings), so name, icon, translations and actions
stay the app's and follow its updates.

- Package entries are rewritten at build time; Flatpak/host file entries at
  runtime by steam-frame-nix-launchers into
  <session.runtimeDir>/steam-frame-nix/applications (tmpfs), on switch, at
  login and when Flatpak installs/updates apps (path unit); the Home Manager
  links point there.
- keyring.{enable,electron} replaces steamFrame.keyring.{flatpaks,programs}
  (clean break: the old options fail with the new form).
- Firefox's launcher is the Flatpak's entry with the profile wrapper in
  front (no --profile for the profile manager); Jellyfin's hardware
  decoding sets flatpakArgs and env.
- cleanup --all removes the tmpfs entries.
- Check on copies of the real Element, KRDC, Firefox, gedit, Jellyfin and
  Claude entries, quoting and generator behaviour.

docs/firefox.md

@@ -14,9 +14,10 @@ browser the portal opens links with the first installed `https` handler
 
 ## What you get
 
-A launcher for the Flathub Firefox Flatpak (`org.mozilla.firefox`, stable;
-install it yourself). The launcher shadows the Flatpak's own entry (same
-ID), so default-browser associations keep working.
+A [launcher](launchers.md) for the Flathub Firefox Flatpak
+(`org.mozilla.firefox`, stable; install it yourself): the Flatpak's own
+entry with a wrapper in front, same ID, so default-browser associations
+keep working.
 
 - **`vrFullscreenFix`** (on): `full-screen-api.ignore-widgets` makes
   fullscreen fill just the window. Not applied in the desktop profile.
@@ -26,11 +27,11 @@ ID), so default-browser associations keep working.
 - **`prefs`:** further `about:config` values for every profile; they can
   also override the fixes above.
 - **`desktopProfile`** (`"desktop"`): in the nested desktop the launcher
-  uses this separate profile (a normal Firefox profile with its own browser
+  uses this separate profile (not for "Open Profile Manager") (a normal Firefox profile with its own browser
   data, created on first use). `null`: the default profile in both sessions.
 - **`defaultBrowser`** (off): the launcher becomes the default for `http`,
-  `https` and `text/html` (Home Manager's `xdg.mimeApps`, which then owns
-  `~/.config/mimeapps.list`).
+  `https` and `text/html` (the launcher's `defaultFor`: Home Manager's
+  `xdg.mimeApps`, which then owns `~/.config/mimeapps.list`).
 
 `prefs` and the fixes are *default* values, not user values: `about:config`
 can still change them per profile, and removing one leaves nothing behind.
@@ -71,9 +72,11 @@ to a store directory; Flatpak mounts it itself, so the sandbox doesn't get
 
 ### Desktop profile
 
-The launcher (a desktop entry with the Flatpak's ID) picks `desktopProfile`
-when started in the nested desktop. The desktop profile undoes the
-fullscreen fix only while its Firefox runs: the launcher links the
+The launcher's wrapper (`modules/firefox/wrapper.nix`) gets the entry's
+`flatpak run …` command line and, in the nested desktop, adds `--profile
+<desktopProfile>` (not to the profile manager action, `--ProfileManager`).
+The desktop profile undoes the fullscreen fix only while its Firefox runs:
+the wrapper links the
 profile's `user.js` to `/app/etc/firefox/steam-frame-nix-desktop-user.js`
 (a sandbox path) right before starting Firefox, waits for it, and once it
 has exited and the profile is no longer in use removes the link and the