Fix boot-critical, session, and UX bugs found in round-3 UX audit

Snapshots panel was non-functional (wrong snapper flag); breadgreet picked
the wrong session .desktop and skipped bos-session's PATH fixup; Calamares
aborted offline installs over an unnecessary packages module; snapshot
rollback silently no-op'd on BOS's pinned-subvolume layout (now points at
grub-btrfs instead); breadcrumbs was configurable but had no daemon to run
it. Also: SUPER+I double-bind, breadpaper/packages panels blocking the GTK
main thread, dead polkit rule, and a sweep of smaller drift (default
sidebar view, stale wording, missing .desktop launchers, wallpaper daemon
not recording its own default).

iso/airootfs/etc/greetd/breadgreet.toml

@@ -0,0 +1,15 @@
+# BOS's real breadgreet config — see breadgreet.example.toml in this same
+# directory for the full set of options with explanations.
+#
+# The one setting that actually matters here: `sessions.default` MUST be
+# "bos", not breadgreet's compiled-in default of "hyprland". The `hyprland`
+# pacman package installs its own /usr/share/wayland-sessions/hyprland.desktop
+# alongside BOS's own bos.desktop, and breadgreet's session picker matches by
+# .desktop file stem — with no override it picks "hyprland.desktop" over
+# "bos.desktop", which skips bos-session's PATH fixup (adds ~/.local/bin for
+# the bakery bread apps; greetd starts no login shell, so /etc/profile.d is
+# never sourced any other way). Confirmed via breadgreet's own test suite
+# (sessions.rs: discover_prefers_configured_default_over_first_entry).
+
+[sessions]
+default = "bos"