Centre modals with a rule about modals, not a list of them

SearchPalette opened pinned to the top of the screen: the CSS named the
screens that centre (`SymbolPalette, StringsPalette, ProjectPalette, …`)
and a new dialog is not on a list nobody remembers to edit. The comment
sitting above that rule — "every #pal-box palette centres, not just the
symbol one" — was the *first* time this happened.

`ModalScreen { align: center middle; }` matches subclasses, so every
dialog inherits it and the next one is centred for free; the eight
per-screen rules that only repeated it are gone. Textual's own Ctrl+P
CommandPalette is a ModalScreen too and wants its stock top alignment, so
it opts out in one visible line rather than by omission.

The `modal_centering` scenario checks both halves: that centring is
expressed as a rule, and that it actually reaches a dialog's laid-out
region (above/below and left/right within a cell).

894 passed, 0 failed.

0 files changed, 0 insertions, 0 deletions