fix: clamp tab index in move_tab to prevent out-of-bounds insert

[enomado]

Summary

When reordering a tab within the same node via drag-and-drop, DockState::move_tab calls remove_tab(src_tab) before insert_tab(index, tab). If the source and destination are the same node, this reduces the tab count by one — making the original destination index potentially out of bounds.

Example: A node has 3 tabs [A, B, C]. Dragging tab B (index 1) to position 3 (the end):

1. remove_tab(1) → node becomes [A, C] (count = 2)
2. insert_tab(3, B) → panic, index 3 > count 2

Fix

Clamp the insertion index to the current tab count after removal. This is safe because inserting at count is equivalent to appending — which is the correct semantic for "move to the end".

Test plan

• Drag a tab to the last position within its own node — no panic
• Drag tabs between different nodes — unchanged behavior
• Drag tabs to split positions — unchanged behavior