Add native themed session locker and Settings integration

docs/locking.md

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+# Native locker
+
+Ferese Lock uses `ext-session-lock-v1`, not a fullscreen application overlay.
+Each output receives its own surface, including outputs added while locked.
+The compositor protects the desktop if the client exits or crashes.
+
+## Preview
+
+```sh
+cargo build --release --locked -p ferese-lock
+target/release/ferese-lock --preview
+```
+
+This is an ordinary, closable window. Submitting the field does not call PAM,
+lock, or unlock anything. Do not type your actual password into a visual preview.
+
+Settings → Lock screen provides clock visibility, date visibility, time format,
+wallpaper dimming and softness, and a safe preview button. Your account picture is loaded from
+AccountsService or `~/.face`, with a user silhouette as a fallback.
+
+The locker reads Ferese's wallpaper, theme colors, typography, and shell radius
+at startup. The wallpaper is softened once and shared across displays. Set softness
+to zero to keep it sharp. Escape clears the password. Caps Lock is shown above authentication
+status. Password verification runs on a worker, and repeated attempts are held
+for two seconds after failure, in addition to delays enforced by PAM.
+
+## Installation and locking
+
+The normal `scripts/install.sh` includes the native binary and installs
+`/etc/pam.d/ferese-lock` if absent. Existing administrator policies are preserved.
+Fedora/Arch installations use `system-auth`; Debian-family installations use
+`common-auth` and `common-account`; the fallback uses the system `login` stack.
+There is no setuid binary, password command argument, or shadow-file reader.
+Both authentication and account validation must succeed. Expired credentials
+must be updated outside the locker. Interactive multi-factor enrollment and
+password changes are not supported by this initial password UI.
+
+```sh
+ferese-lock                # returns after confirmation, UI stays running
+ferese-lock --foreground   # stays attached until unlock
+```
+
+The `swayidle -w` example in [Configuration](configuration.md#login-items-and-locking)
+can invoke the default command. Auto-locking is opt-in. This change does not
+install or start an idle daemon, replace your login manager, or change the Control Center's
+suspend action. Test real password authentication inside a nested Ferese session
+before enabling this on your desktop.
+
+## Security and recovery
+
+Never interpret the preview as proof of a locked desktop. A missing PAM policy
+or unsupported compositor produces an error before a lock request. A failure
+or authentication-worker panic never authorizes unlock. A crash after the lock
+request may leave the session permanently locked; end that session from another
+TTY if necessary. Do not restart the compositor to test crash behavior on a live
+session with unsaved work.
+
+The pinned libcosmic backend emits `Locked` twice: once when requesting a lock
+and once on the actual protocol callback. The startup handshake waits for the
+second notification. Re-audit this behavior whenever upgrading libcosmic. The
+parent times out with an error after 15 seconds and leaves the UI running; it
+never reports success merely because the window appeared.
+
+Validation references: [session-lock protocol](https://wayland.app/protocols/ext-session-lock-v1),
+[PAM authentication](https://man7.org/linux/man-pages/man3/pam_authenticate.3.html),
+and [PAM account checks](https://man7.org/linux/man-pages/man3/pam_acct_mgmt.3.html).