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 :(

[umop]

Very odd. My upstream baseline margin looked different than yours. Perhaps OS differences? I wasn't able to view your screenshots at full resolution (GitHub isn't letting me view them outside of the inline preview), so I estimated the target margin by measuring pixels from the inline images:

Your before (upstream): ~23px bottom margin
Your after (my PR): ~30px bottom margin
My build (macOS): ~35px bottom margin
Cross-platform scaling factor: 35/30 ≈ 1.17, so targeting 23 px on your end → 23 × 1.17 ≈ 27px on mine. That's a delta of -8px from what I was seeing (35 → 27), which translates to 30 - 8 = 22 in the .ui file.

I changed the bottom margin from 30 to 22. Should be much closer to the upstream baseline. Let me know if it's still off on your end and I can tweak further.

Befores on my end (note feature not added):

Afters:

[luke-jr]

There are infinite OS/style combinations; trying to special-case one or two of those isn't the right approach.

[umop]

Sorry, what I meant is that I reduced the margin by what seemed to be the correct amount per your screenshots, so the margin should be correct now. I amended the previous commit to avoid commit cruft per your previous comment about squashing the commits down to one, so maybe it wasn't clear that a change was made.