fix(ui-primitives): keep two-digit source markers out of the scroll clip

Making `.sources` a scroll container turned its `padding-left` from spacing
into a correctness constraint. A scroll container clips inline-start overflow
with no way to scroll it back, and `::marker` is right-aligned to the content
edge, so past nine sources the markers rendered as `0.` and `1.` where `10.`
and `11.` belonged.

`searchMaxResults` is an unbounded positive integer, so size the padding in
`em` against the list's own font to hold a three-digit marker. The browser e2e
measures a `999. ` marker in that inherited font and requires the computed
padding to be at least that wide, pinning the room against the widest marker
rather than one fixture's source count.

packages/client/ui-primitives/src/WebBlock.tsx

@@ -11,7 +11,9 @@
 // plain text. Geometry, radius, and fonts mirror CodeBlock/TerminalBlock so a
 // web card reads as one family with them; the whole source list renders inside a
 // fixed-height scroll container (its `.sources` max-height), so a long list
-// scrolls in place rather than growing the card. The card draws every source the
+// scrolls in place rather than growing the card — and that container's
+// `padding-left` must stay wide enough for the widest `<li>` marker, since a
+// scroll container clips inline-start overflow irrecoverably. The card draws every source the
 // view carries: the tool already cut the list to its source cap, and `truncated`
 // reports that cut. A content-only transform downstream of the tool — spill-policy
 // replacing an oversized result's text while leaving its presentationMeta whole —