M1: make mount/unmount idempotent

mount now detects already-unlocked and already-mounted devices, returning
the existing mount point instead of failing. unmount handles already-locked
devices gracefully and skips unmount if not mounted before locking.

Adds IsMounted helper to udisks client. Updates PLAN.md with refined
v1.0.0 milestones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/udisks/client.go

@@ -97,6 +97,18 @@ func (c *Client) MountPoint(dev *BlockDevice) (string, error) {
 	return string(bytes.TrimRight(mountPoints[0], "\x00")), nil
 }
 
+// IsMounted returns the mount point and true if the device is mounted.
+func (c *Client) IsMounted(dev *BlockDevice) (string, bool) {
+	if !dev.HasFilesystem {
+		return "", false
+	}
+	mp, err := c.MountPoint(dev)
+	if err != nil || mp == "" {
+		return "", false
+	}
+	return mp, true
+}
+
 // DeviceAtPath returns the block device at the given D-Bus object path.
 func (c *Client) DeviceAtPath(path dbus.ObjectPath) (*BlockDevice, error) {
 	devices, err := c.listBlockDevices()