fix(packaging): create the punktfunk group everywhere the udev rule needs it

60-punktfunk.rules chgrp's the usbip vhci attach/detach nodes to a dedicated
`punktfunk` group (security-review 2026-08-05 M-4: writing `attach` materialises
an arbitrary emulated USB device, so it must not ride on `input`). Four of the
six install paths shipped that rule in 0.25.0 without ever creating the group.
chgrp then failed, the nodes stayed root:root 0644, and the virtual Steam Deck
pad silently never attached — while `usermod -aG punktfunk` failed outright with
"group 'punktfunk' does not exist".

Affected and fixed:

  * arch  — post_upgrade() called only _ensure_update_group, so every box that
            reached 0.25.0 by `pacman -Syu` missed it; post_install was correct.
  * nix   — no users.groups.punktfunk at all, though host.users' own description
            already promised the usbip/vhci pad. Declares it now and adds
            host.users to both groups.
  * bazzite sysext — a group is host state and cannot ride an image, and the
            deb/rpm scriptlets that would create it never run there.
  * steamdeck install.sh/update.sh — handled `input` only. Both now create the
            group and join it: running that script IS the statement "make my
            Deck a host with native pad passthrough".

deb and rpm were correct throughout (one postinst/%post for install + upgrade).

Also on the Deck path: web.env secret hygiene. install.sh's `chmod 600` sat
inside the create-only branch despite a comment calling it "the idempotent belt
for a pre-existing file", and update.sh never touched the config dir at all — so
an install set up once and only updated since kept web.env world-readable
(0644) with the console password and session secret in it. Both scripts now
harden ~/.config/punktfunk to 0700 and web.env to 0600 on every run, and say so
loudly, because a chmod does not un-leak an already-readable secret: the
password still needs rotating.

Both group blocks are `if ensure_group ...` rather than `ensure_group || true`:
a failed groupadd must not fall through to a usermod against a nonexistent
group, which under `set -e` aborted install.sh after the long build and
update.sh before the service restart (verified: exit 6, no restart).

Docs: the group is now documented where people actually look — the per-distro
guides, install.md, steamos-host.md, a new troubleshooting entry for "pad
arrives as an Xbox 360 controller", and the uninstall pages. The 0.25.0 notes
gain the "group does not exist" caveat and turn the password bullet from
"consider rotating" into a real instruction, and CHANGELOG records the known
issue against the breaking change that introduced it.

Verified: bash -n on all four scripts; the arch scriptlet's post_upgrade driven
in a container (creates the group, idempotent on re-run); the ensure_group
helper and both membership branches, including a control that reproduces the
original bug (chgrp to a missing group leaves the node root:root 0644); the
find -perm /0077 probe across 0644/0640/0604/0600/0400 on GNU findutils;
`nix flake check --no-build` (the exact CI gate) and a NixOS eval showing
alice.extraGroups == ["input","punktfunk"]; docs-site build + typecheck.

docs/releases/v0.25.0.md

@@ -16,12 +16,12 @@ This is the largest release so far — close to four hundred changes. The short
 
 Most people need to do nothing. Check this list if any of it applies to you.
 
-- **Linux, if you use the virtual Steam Deck controller: join a new group.** That permission used to ride on `input`, which every gamepad guide tells you to join — but it can emulate arbitrary USB hardware, so it now has its own. Run `sudo usermod -aG punktfunk "$USER"` and log back in, or the virtual Deck pad stops attaching. Ordinary virtual gamepads are unaffected, and you should only join this group on a machine you trust.
+- **Linux hosts, if you use the virtual Steam Deck controller: join a new group.** That permission used to ride on `input`, which every gamepad guide tells you to join — but it can emulate arbitrary USB hardware, so it now has its own. Run `sudo usermod -aG punktfunk "$USER"` and log back in, or the virtual Deck pad stops attaching. Ordinary virtual gamepads are unaffected, and you should only join this group on a machine you trust. This is a host-side step: a Linux machine you only *stream from* needs nothing. **If that command reports `group 'punktfunk' does not exist`**, you are on one of the install paths that shipped 0.25.0 without creating it — an Arch box upgraded rather than freshly installed, NixOS, the Bazzite sysext, or a Steam Deck source install. `sudo groupadd --system punktfunk` first, then the `usermod`; a later update creates it for you.
 - **Add-on interfaces moved to their own port (47993).** An existing firewall rule will not pick it up when you upgrade, and the symptom is a blank panel where the add-on's interface should be. On Linux the package prints the exact command — `sudo ufw app update punktfunk-web && sudo ufw reload`, or a firewalld reload. With Docker, publish `47993` as well. If you reach your console over a self-signed certificate, your browser needs to trust the new port once; the console shows a card with a link that does it.
 - **Windows hosts now need Steam installed** for streamed audio — it never has to run. Without it the host streams video only, and picks the drivers up on its own if you install Steam later. Two new devices, "Punktfunk Speakers" and "Punktfunk Microphone", will appear in your sound settings; that is this feature working. If you already have VB-CABLE, leave it — it still works as a fallback and is not removed.
 - **Saving a game with a custom launch command asks for your console password again**, and add-ons may no longer set launch commands at all. A third-party add-on that did will need updating by its author.
 - **A fresh install now runs the add-on runner by default.** Upgrades are untouched — if you switched it off, it stays off.
-- **If you set up a Steam Deck with the install script, consider rotating your console password.** It was written to a world-readable file; that is fixed.
+- **If you set up a Steam Deck with the install script, rotate your console password.** It was written to a world-readable file, so any local account could read it — and fixing the permissions does not un-share a password that was already readable, which is why this one is worth actually doing rather than considering. New installs are written correctly. On an existing one, re-run `install.sh` or `update.sh` to tighten the file, then change `PUNKTFUNK_UI_PASSWORD` in `~/.config/punktfunk/web.env` and `systemctl --user restart punktfunk-web`.
 - **If you play with motion controls, your aim sensitivity will change.** The gyro pipeline was wrong at every stage and is now measured against a real controller, so the numbers moved: a controller presented to games as a DualShock 4 was reporting motion **forty times too fast**, and a PlayStation pad plugged into an Android phone was reporting about **30% short**. If you turned a game's sensitivity down or up to cope, set it back. The Android case is the one people plausibly tuned around — that aim now needs a *higher* in-game sensitivity than you are used to.
 
 ## New