fix: calculate cell size before presenting gtk window (#10459)

Fixes #7937

Added `computeInitialSize` to GTK `Surface` and call it in GTK
`Application` before the first `present()`, so the window manager
centers the correct size on initial show.

The issue occurs because the core `Surface.recomputeInitialSize()` runs
only after the renderer is initialized. In GTK, the `GLArea` isn’t
realized until after `present()`, so the initial size arrives too late
for WM centering.

**Limitations**: when we precompute size before `present()` we do not
have access to padding, so the sizing will be very slightly off... but
since it is only off a few pixels I was unable to tell visually that it
wasn't perfectly centered.

**Other thoughts**: I was hesitant to make changes to core `Surface`
because the issue is Linux-specific, but it may make sense to extract a
helper from `recomputeInitialSize` to avoid duplicating the sizing math.

**AI Disclosure:** I used AI to explore the project, help with any
language / API questions (I've never used zig before and rarely use
gtk), and make implementation suggestions.

src/Surface.zig

@@ -46,8 +46,8 @@ const Renderer = rendererpkg.Renderer;
 /// being resized to a size that is too small to be useful. These defaults
 /// are chosen to match the default size of Mac's Terminal.app, but is
 /// otherwise somewhat arbitrary.
-const min_window_width_cells: u32 = 10;
-const min_window_height_cells: u32 = 4;
+pub const min_window_width_cells: u32 = 10;
+pub const min_window_height_cells: u32 = 4;
 
 /// The maximum number of key tables that can be active at any
 /// given time. `activate_key_table` calls after this are ignored.