fix: save typed credentials only after the connection succeeds, and re-ask when a saved one is rejected

A password typed at a 'saved' prompt was stored before the server
accepted it, so a typo was persisted and silently retried on every
reconnect. Typed values are now held per attempt and written once the
session connects; when the server refuses a saved credential the next
attempt asks again and the new value replaces it on success. A store
failure (keychain refused, vault locked) now shows a toast instead of
being swallowed, and reconnect prompts wait 5 minutes like the first
connect (SEC-20261012-015, F1, F2, F20).

rust/core/src/secrets.rs

@@ -269,10 +269,6 @@ impl Secrets {
     }
 
     /// Stores where possible: the keychain, else the vault if it's unlocked.
-    pub fn set(&self, key: &str, value: &str) {
-        let _ = self.set_checked(key, value);
-    }
-
     pub fn set_checked(&self, key: &str, value: &str) -> Result<()> {
         if self.keychain_ok && self.keychain.set(key, value).is_ok() {
             return Ok(());