fix(ui): move Compose back, drop the bar below the header, size its icons

Three corrections from looking at the built bar.

Compose returns to the main toolbar. The split this was built to, "about a
message" against "about the list", does not survive contact: what matters is
what the action NEEDS. Reply and Forward are meaningless without a message on
display, while Compose needs none and is disabled only when no account can
send. So the pane's bar holds exactly the two actions that depend on what it
is showing, and Compose sits with the window-wide ones.

The bar moves below the subject and details rows, directly above the web
view. At the top of the pane it read as window chrome rather than as
belonging to the message. The transient notice bars stay above it: they
explain the message rather than offer an action on it.

Its icons were the style's own default, 16px, which is tiny beside a 32px
toolbar. They are now 7/8 of toolbar_icon_size, which is the 28 the user
asked for at their 32, derived rather than hardcoded so the relation holds
if that key changes. The test asserts the relation as well as the value,
since a bare 28 would stop meaning anything the moment the key moved.

m_headerLabel gains an object name so the placement test can find the row it
must sit below.

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