fix(settings): centre the content column and quiet the dividers

[winetree94]

Why

Two things the settings surface still got wrong after #77 and #83.

The content column sat against the leading edge. The width cap keeps a label and its control near each other, but left-aligning the capped column strands it against one side of a wide window with a growing void beside it. It is centred now. Below the cap the column fills its pane and the change is a no-op — which is what a list-detail detail pane wants — and both cases are pinned by tests.

Dividers were drawn at the default TRSeparator variant, which resolves to controlBorder: the weight a control draws at, not a surface. Beside a TRCard outlined in border that read as a brighter line running through a quieter frame — 163 against 64 in dark, 115 against 212 in light. Every divider in settings is muted now, so a card and the lines inside it close on the same colour.

Note

This commit was written before #83 merged but could not be pushed at the time: the branch was locked while queued for merging, and I then lost it to a hard reset. Recovered from the object store and rebased onto current main; the goldens are regenerated on top of #84.

The divider change becomes a no-op once tinyrack-net/design#444 lands, since that unifies border and controlBorder. It is kept because it states the intent — a divider is a surface — and stays correct if the two ever diverge again.

Verification

• dart run melos verify — 12/12.
• 460 widget and unit tests.
• Goldens regenerated; the centring is visible on General, Daemons, Advanced, and Provider, and correctly absent on the list-detail detail panes, which are narrower than the cap.