qt: Toggle banned peers visibility

[umop]

Closes #185
Adds a Hide / Show button on the banned peer list table which shows / hides banned table if visible. Default is to show. Also persists the state in QSettings.

Also cleaned up some indentation and closing tags in the debugwindow.ui

There don't appear to be existing automated tests for individual Peers tab UI elements or visibility toggles. The existing Qt tests focus on RPC command parsing, wallet operations, and high-level integration testing rather than specific widget interactions.

Manually tested:

• Ban a peer -> verify section appears with Hide button
• Click Hide -> verify table collapses, button shows "Show"
• Click Show -> verify table reappears, button shows "Hide"
• Close and reopen Node window -> verify state persists
• Unban all peers → verify section (heading + button + table) disappears
• Button vertical alignment with "Banned peers" label
• No impact on other tabs (Info, Console, Graph)
• No extra spacing when table is hidden
• Waiting for expiry and created Banned peer remains after expiration. #273 which remains (banned peers remain in list after expiry).

Showing:

Hidden:

[luke-jr]

This change should be a single commit, with a subject like "GUI/Peers: Add button to hide/show ban table" and details after a blank line.

[luke-jr]

This is so close, but the Peers tab without any bans now has an extra padding at the bottom

[umop]

I set the bottom margin to 30 to match the current ui at HEAD. There is still space there, though (there always was), so if we want to match other tab layouts, we can reduce that to a smaller value:

Current and old Peers tab:

Console tab:

[luke-jr]

Before:

After:

Before:

After:

Margins still very different :(