Refine notification center and configure native notifications

Keep notification actions and dismiss controls visible, compact and animate history, and add themed icon controls and grouping.

Include notification settings, session integration and documentation, on-demand workspaces, and performance baseline notes.

docs/notifications.md

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+# Notifications
+
+Ferese Shell provides themed notification cards and an in-session history panel.
+Cards appear beneath the bar on the focused output and follow the selected theme.
+Click the notification icon in the bar to open history, toggle Do Not Disturb, or
+clear notifications. Escape or the close button closes the history panel.
+
+Notifications use the standard `org.freedesktop.Notifications` D-Bus interface.
+Existing apps and `notify-send` work without Ferese-specific commands:
+
+```sh
+notify-send --app-name=Ferese --icon=dialog-information "Hello" "Your desktop notification"
+```
+
+Up to three popup cards are visible at once. History keeps the most recent 100
+notifications in memory and clears when the shell exits. Hovering a popup pauses
+its timer. Critical notifications bypass Do Not Disturb and do not expire unless
+the sender supplies a timeout. App action buttons emit the standard action signal;
+a replacement updates the same notification ID. Transient messages are removed
+from history when they close.
+
+Settings → Notifications controls popup visibility, the initial Do Not Disturb
+state, and the default timeout. The bar's Do Not Disturb toggle is temporary;
+changing the Settings option saves the preference for future sessions. Apps can
+request their own timeout, including no timeout.
+
+```kdl
+notifications {
+    show-popups #true
+    do-not-disturb #false
+    timeout-ms 6000
+}
+```
+
+The timeout must be between 1000 and 30000 milliseconds. Hiding popups still
+records notifications in history. Appearance follows the Ferese theme.
+
+Only one notification server can own the D-Bus interface. The Ferese session
+launcher stops the SwayNotificationCenter user service before starting its shell.
+For a manually started shell, stop that service with `systemctl --user stop
+swaync.service`. If another server already owns the interface, Ferese leaves it
+running and logs that native notifications are unavailable.
+
+This implementation supports plain text, app icons, urgency, transient/resident
+hints, actions, replacements, and close signals. It does not advertise rich body
+markup, embedded image data, notification sounds, or history persistence.