feat(sharing,gui,cli): change public links and show folder sizes

The share dialog gets Link settings under an existing link: role, password
and expiry apply at once and keep the URL. The daemon fills in the options a
change keeps from the link as it stands, and refuses to keep a password it
cannot read rather than dropping it. pdfs public-link update does the same
from a terminal, and pdfs members shows the expiry.

Details shows a folder's size and item count as Proton Drive counts it,
trash included, and pdfs du prints it. Both are refused offline and for a
folder that is not on Drive yet.

Needs proton-drive-rs with update_public_link, get_folder_size and
PublicLink::custom_password.

docs/SHARING.md

@@ -33,6 +33,12 @@ In **Share…**, create a public link. Anyone with the link can open the item. Y
 link with a password and let it expire on a date. From a terminal, use `pdfs public-link create`
 and `pdfs public-link remove`.
 
+To change a link later, open **Link settings** under it. You can switch it between Viewer and
+Editor, set, change or remove its password, and change or remove its expiry. The address stays
+the same, so everyone who has the link keeps it. From a terminal, use `pdfs public-link update`;
+`pdfs members` shows the link's id. A link made long ago by an older Proton app may not be
+changeable; remove it and create a new one instead.
+
 ## See what you share
 
 **Shared by me** lists everything you share. Filter it by links, by people, or by expired links.