Add VNC-only scale-to-fit vs actual-size display toggle

VNC has no equivalent of RDP's MS-RDPEDISP to request a different
resolution from the guest, so a high-resolution remote desktop
previously always got shrunk to fit the window, making text
illegible. Adds a per-tab "Display Mode" choice (tab-bar right-click)
between the existing scale-to-fit behavior and a new actual-size mode
that renders the framebuffer at its native pixel size inside a
QScrollArea. Reuses VncDisplayWidget's existing scale-to-fit render
math unchanged -- it degenerates to an exact 1:1 mapping once the
widget is fixed to the remote's own size. Persisted like the terminal
theme preference.

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

docs/PROGRESS.md

@@ -133,6 +133,15 @@ Delivered:
 - Verified live against a real, independently implemented VNC server
   (TightVNC on Windows): connect with VNC Authentication, correct
   framebuffer dimensions and pixel data, clean disconnect, reconnect
+- Per-tab VNC-only display mode toggle (tab-bar right-click ->
+  `Display Mode`): `Scale to Fit` (default, matches RDP's behavior) or
+  `Actual Size (Scrollbars)` -- renders the remote framebuffer at its
+  native pixel size inside a `QScrollArea` so text isn't shrunk, at the
+  cost of needing to scroll to see the whole screen. Reuses
+  `VncDisplayWidget::renderRect()`'s existing scale-to-fit math unchanged:
+  it degenerates to an exact 1:1 mapping once the widget's own bounds are
+  fixed to the remote's size, so no separate rendering path was needed.
+  Persisted across sessions like the terminal theme preference.
 
 Known gaps (explicit scope decisions, not oversights -- see issue #3 for
 follow-up tracking):
@@ -141,9 +150,10 @@ follow-up tracking):
   server -- only standard VNC Authentication (type 2) and no-auth (type 1)
 - Raw + CopyRect encodings only -- no Hextile/ZRLE/Tight compression, so
   bandwidth usage is higher over slow links than a full VNC client
-- No dynamic resize (connects at the server's native resolution, scaled to
-  fit locally -- the same way `RdpDisplayWidget` already renders
-  regardless of server resolution, so not a UX regression vs. RDP)
+- No dynamic resize (connects at the server's native resolution; the
+  `Scale to Fit`/`Actual Size` toggle changes how that fixed resolution is
+  displayed locally, not what resolution is requested from the guest --
+  VNC has no equivalent of RDP's MS-RDPEDISP for that)
 - No remote cursor shape sync (local default cursor only)
 - No clipboard sync