fix(client): report a directory result under the button that caused it

The message was the first thing in the section, wedged between the intro and
the table header — above everything the eye has already moved past by the time
it appears. It goes last now, under Add directory.

And it was a `settings-hint`: dim grey body text. So "two roots would both be
called uploads" read as a footnote about the section rather than as the reason
nothing happened. A refusal is styled as one and carries `role="alert"`, so it
is announced rather than only drawn; a success stays quiet.

Measured in a real Electron window rather than assumed — the failing add
driven through the typed-path form, then the message's box compared against the
table's and the button's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pvMdvLBG92jyhvD5pD6us

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