Let the top level be chosen as the library root

Confirming "/" in the folder picker set an empty root, which the launch
model read as no root at all: "Open library" stayed disabled after the
question had plainly been answered, and a folder library — whose folder
is the whole library — could never be opened without first descending
into a subfolder of it. The empty string was carrying two meanings.

Record the choice as its own fact on the account (`root_chosen`, defaulted
so existing configuration loads unchanged), treat a folder endpoint as
chosen by definition, and let the launch screen say so: a folder is shown
as a LIBRARY rather than an ACCOUNT, the second question becomes an
optional "scan only a subfolder", and the library header names the folder
instead of calling it "· whole account".

core/dr-sync/src/account.rs

@@ -127,6 +127,19 @@ pub struct Account {
     #[serde(default)]
     pub root: String,
 
+    /// Whether [`root`](Self::root) has been chosen at all.
+    ///
+    /// An empty `root` is two different things: nothing picked yet, and the
+    /// endpoint itself picked on purpose — a user who keeps everything at
+    /// the top level, or a folder library, which is its own root. The string
+    /// cannot tell them apart, and reading empty as "not chosen" meant the
+    /// top level could be confirmed in the picker and still not open. So the
+    /// fact is recorded separately. Defaulted, so an account written before
+    /// it existed loads as it always did: a non-empty root is chosen by
+    /// virtue of being there, and an empty one asks again.
+    #[serde(default)]
+    pub root_chosen: bool,
+
     /// Which formats the scan looks for (the tick-boxes).
     #[serde(default)]
     pub formats: Vec<String>,
@@ -149,6 +162,7 @@ impl Account {
             login: String::new(),
             user_id: String::new(),
             root: String::new(),
+            root_chosen: false,
             formats: Vec::new(),
             last_scan: None,
         }