AvailableMeters inconsistent behavior on "Add Lt", "Add Rt" UI focus

[Explorer09]

This is a follow up of #1942.

The "Add left" and "Add right" functions in the "Available meters" have inconsistent behaviors on which panel should get focus after the add.

I have a video capture for demonstration:

2026-04-07.7.14.22.mov

1. Go to "Setup" -> "Meters"
2. Press Right key repeatedly until the "Available meters" is in focus.
3. Select a meter to add and press F5. Observe the "Available meters" panel remains focused.
4. Select another meter to add and press F6. Observe the focused panel shifts to the "Column 2" panel.

The expected result is consistent behavior in steps 3 and 4.

I can point out that the problem likely happens in the two different result values in AvailableMetersPanel_eventHandler():

htop/AvailableMetersPanel.c

Line 65 in 6b15bd7

result = HANDLED;

         result = HANDLED;

htop/AvailableMetersPanel.c

Line 76 in 6b15bd7

result = (KEY_LEFT << 16) | SYNTH_KEY;

         result = (KEY_LEFT << 16) | SYNTH_KEY;

[fasterit]

It has always been like this. On the right column we know we need to go one left. On the left column we would need to check how many columns there are. Nobody ever invested the effort to do this.