Fix size_t underflow in TabsBase::remove bounds check

[94xhn]

Problem

`TabsBase::remove(std::size_t index)` checks bounds like this:

```cpp
if (index > m_tabs.size() - 1)
return false;
```

When `m_tabs` is empty, `m_tabs.size() - 1` underflows (unsigned) to `SIZE_MAX`, so the check never returns `false` and execution falls through to:

```cpp
m_tabs.erase(m_tabs.cbegin() + static_caststd::ptrdiff_t(index));
```

which erases from an empty vector at an out-of-range position, invoking undefined behavior (crashes both in a plain optimized build and under `_GLIBCXX_DEBUG`).

Both `Tabs` and `VerticalTabs` inherit `remove()` from `TabsBase` without overriding it, and the header doc comment for this function promises "false is returned when the index was too high" — a promise this code breaks precisely in the empty-container case.

Fix

Rewrite the bounds check as `index >= m_tabs.size()`, which is safe for the empty case and matches the equivalent checks already used elsewhere in the codebase (e.g. `BoxLayout::remove`, `ListBox::removeItemByIndex`, `TabContainer::removeTab`).

Testing

Extracted the exact erase/bounds logic into a standalone reproduction: calling `remove(0)` on an empty container segfaults with the old `> size() - 1` check (both under `-O2` and under `_GLIBCXX_DEBUG`), and correctly returns `false` with no crash after applying this fix.