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.module.css

@@ -33,10 +33,19 @@
    renders here; a max-height caps the card so a long list scrolls in place
    rather than growing the card unbounded. The height is a design constant of the
    card's geometry, not a deployment choice, so it lives here rather than a plugin
-   config field. */
+   config field.
+
+   `overflow-y` makes this a scroll container, which also clips inline-start
+   overflow: a marker wider than `padding-left` loses its leading digits with no
+   way to scroll them back. Markers are right-aligned to the content edge, so the
+   padding must fit the widest one the list can produce. `searchMaxResults` is an
+   unbounded positive integer, so the padding is sized in `em` — against this
+   element's own font, the one a marker inherits — to hold a three-digit marker
+   (`999. ` measures 2.35em in the app font stack) plus the gap the one-digit
+   case already had. */
 .sources {
   margin: 0;
-  padding-left: 20px;
+  padding-left: 2.5em;
   display: flex;
   flex-direction: column;
   gap: 10px;