Files: a tile's name keeps its ends; the ellipses share one line cutter

gpui's line_clamp wraps once and lays the rest of a long name on the
second line, which a centered tile then clipped at both ends. A tile's
name is now one line, cut in the middle so the front and the extension
stay. MiddleEllipsis draws like Ellipsis: in the text style around it,
sized by its text, so a short name centers; the two share one cutter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

TASKS.md

@@ -321,3 +321,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T26d | 2 | PASS | |
 | T26e | 1 | FAIL | an option list had no width limit, so a long device name ran it past the window |
 | T26e | 2 | PASS | |
+| T27a | 1 | FAIL | a tile's long name leaned on gpui's line_clamp, which cut its second line at both ends and lost the extension |