feat: add GTK keybinds (matching the idiomatic Linux convention used by Firefox, GNOME Terminal, and VSCode) for move_tab (#12458)

> Re-submitting #11857, which was auto-closed pre-vouch. I'm now in the
vouched list.

## Summary

Adds default keybinds for `move_tab` on GTK/Linux, matching the
idiomatic Linux convention used by Firefox, GNOME Terminal, and VSCode:

- **`Ctrl+Shift+PageUp`** → `move_tab:-1` (move tab left)
- **`Ctrl+Shift+PageDown`** → `move_tab:1` (move tab right)

To free these keys, `jump_to_prompt` is reassigned to `Ctrl+Shift+Arrow
Up/Down`, which:
- Mirrors the macOS default (`Cmd+Shift+Arrow Up/Down`)
- Is currently unbound on GTK
- Maintains directional consistency (up = previous, down = next)

The new `move_tab` bindings use `putFlags` with `performable: true` to
match the pattern used by other tab actions (`previous_tab`,
`next_tab`).

Closes #4998

## Changes
- `src/config/Config.zig`: Reassign `jump_to_prompt` from
`Ctrl+Shift+PageUp/PageDown` to `Ctrl+Shift+Arrow Up/Down` (GTK only)
- `src/config/Config.zig`: Add `move_tab` default keybinds as
`Ctrl+Shift+PageUp/PageDown` (GTK only)

## Test plan
- [ ] Build on Linux/GTK: `zig build`
- [ ] Verify `Ctrl+Shift+PageUp` moves current tab left
- [ ] Verify `Ctrl+Shift+PageDown` moves current tab right
- [ ] Verify `Ctrl+Shift+Arrow Up` jumps to previous prompt
- [ ] Verify `Ctrl+Shift+Arrow Down` jumps to next prompt
- [ ] Verify `Ctrl+PageUp/PageDown` still switches tabs (unchanged)
- [ ] Verify macOS keybinds are unaffected

src/config/Config.zig

@@ -6766,15 +6766,29 @@ pub const Keybinds = struct {
             // Semantic prompts
             try self.set.put(
                 alloc,
-                .{ .key = .{ .physical = .page_up }, .mods = .{ .shift = true, .ctrl = true } },
+                .{ .key = .{ .physical = .arrow_up }, .mods = .{ .shift = true, .ctrl = true } },
                 .{ .jump_to_prompt = -1 },
             );
             try self.set.put(
                 alloc,
-                .{ .key = .{ .physical = .page_down }, .mods = .{ .shift = true, .ctrl = true } },
+                .{ .key = .{ .physical = .arrow_down }, .mods = .{ .shift = true, .ctrl = true } },
                 .{ .jump_to_prompt = 1 },
             );
 
+            // Move tab
+            try self.set.putFlags(
+                alloc,
+                .{ .key = .{ .physical = .page_up }, .mods = .{ .shift = true, .ctrl = true } },
+                .{ .move_tab = -1 },
+                .{ .performable = true },
+            );
+            try self.set.putFlags(
+                alloc,
+                .{ .key = .{ .physical = .page_down }, .mods = .{ .shift = true, .ctrl = true } },
+                .{ .move_tab = 1 },
+                .{ .performable = true },
+            );
+
             // Search
             try self.set.putFlags(
                 alloc,