feat(files): comment on selection in read-only markdown previews

Selecting text in a rendered markdown preview shows a Comment pill;
attaching stores a file-quote context draft carrying the file path, the
selected fragment (not whole lines), the user's comment, and a
best-effort source line range resolved by anchoring the fragment's
first and last lines in the raw content — a partially located fragment
gets no range rather than a misleading one. The fragment stays
highlighted while the comment input is open, using the selection
overlay rects shared with chat quote comments, and the preview's
native selection color now matches chat messages. file-quote flows
through the same context contract: composer chip previews, the message
context card, and the metadata round-trip.

packages/ui/src/index.css

@@ -76,7 +76,9 @@ button,
 }
 
 :root.light .message-content-text::selection,
-:root.light .message-content-text ::selection {
+:root.light .message-content-text ::selection,
+:root.light .oc-file-preview::selection,
+:root.light .oc-file-preview ::selection {
   background: color-mix(in srgb, var(--interactive-border-focus) 18%, transparent);
   color: var(--surface-foreground);
 }