Allow blank usernames, asking for one at connect time instead

Closes #21. SSH and RDP profiles previously hard-required a username
to even save the profile; that validation is dropped, and
SessionTab::requestConnectOptions() now prompts for it at connect
time when blank, reusing the existing password-prompt bar in
unmasked mode -- the same pattern already used for a blank password.

VNC's username is trickier: most VNC servers never use one (plain VNC
Authentication and no-auth don't), only the two Apple auth schemes
(security types 30/33) do, and which auth method gets used isn't known
until mid-connection, after the server's security-type list has been
negotiated -- too late for the pre-connect prompt SSH/RDP uses. Adds a
new async request/response pair to SessionBackend, usernameRequested()
signal / provideUsername() slot, mirroring the existing SSH host-key-
confirmation pattern. VncSessionBackend pauses its state machine right
before computing an Apple-auth response if no username is available --
without consuming the already-buffered prime/host-key bytes, so
resuming re-parses them identically -- emits the request, and resumes
via provideUsername(). Cancelling (or submitting blank) fails the
connection cleanly instead of sending Apple auth an empty username.

The username is kept on the tab's in-memory profile copy for its
lifetime, not written back to the saved profile, matching how
passwords are already handled.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/session_tab.h

@@ -91,6 +91,7 @@ signals:
     void requestMouseButtonEvent(int x, int y, int button, bool pressed);
     void requestMouseWheelEvent(int x, int y, int deltaX, int deltaY);
     void requestSetClipboardText(const QString& text);
+    void requestProvideUsername(const QString& username);
 
 private slots:
     void onBackendStateChanged(SessionState state, const QString& message);
@@ -98,6 +99,7 @@ private slots:
     void onBackendConnectionError(const QString& displayMessage, const QString& rawMessage);
     void onBackendOutputReceived(const QString& text);
     void onBackendHostKeyConfirmationRequested(const QString& prompt);
+    void onBackendUsernameRequested(const QString& prompt);
     void onBackendRemoteClipboardTextChanged(const QString& text);
     void onSystemClipboardChanged();
 
@@ -149,7 +151,8 @@ private:
     void setupUi();
     void requestConnectOptions(std::function<void(std::optional<SessionConnectOptions>)> callback);
     void showPasswordPrompt(const QString& labelText,
-                            std::function<void(std::optional<QString>)> callback);
+                            std::function<void(std::optional<QString>)> callback,
+                            bool maskInput = true);
     void hidePasswordPrompt();
     bool validateProfileForConnect();
     void appendEvent(const QString& message);