feat: refactor the Skills detail pane into collapsible sections (#2263)

The pane was one scrolling column: SKILL.md sat below the manifest behind a
"View SKILL.md" button, the frontmatter pushed it further out of view, and
nothing could be put away.

It is now a flex column that does not scroll as a whole. Conformance,
Resources, Frontmatter and Skill Resource are four collapsible sections of one
`disclosure` accordion, and the file viewer claims the remaining height and
scrolls internally.

The viewer's flex-basis is 0, not auto, and that is load-bearing: `auto` makes
an item's basis its content height, and a rendered document is ~1,795px for the
data-analysis fixture. That basis joined the sum the container distributes,
over-constraining the column so the shrink factors crushed the siblings —
Conformance to a 2px panel, Resources to zero — with their contents spilling
over the headers below. Collapsing the viewer removed the giant basis, so the
symptom appeared on collapse-then-reopen. Open sections also carry a `mih`
floor, since the disclosure CSS sets `min-height: 0` on an active item.

Frontmatter now follows the displayed file rather than the selected skill.
Both halves come from one `splitSkillFile` call, so the section and the viewer
cannot disagree, and a file with no frontmatter renders no section at all.

Also: SKILL.md loads on selection (a `resources/read` is not a load under
SEP-2640), the manifest highlights on hover with each URI a button that swaps
the viewed file, both whole-skill actions moved to the pane header, and the
Conformance badge goes yellow for a warnings-only entry instead of green.

Flat CSS properties moved out of call sites into theme variants (ThemeText
gains four; a new ThemeTable takes the manifest font size).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R4441JdNLZerCVM7mDDSfB
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/theme/Table.ts

@@ -0,0 +1,14 @@
+import { Table } from "@mantine/core";
+
+export const ThemeTable = Table.extend({
+  // The Skills resource manifest (#2263). `striped`, `withTableBorder`,
+  // `highlightOnHover` and `verticalSpacing` are Mantine's own API and stay
+  // props at the call site; the font size is a flat CSS property, so it lives
+  // here rather than as an `fz` prop on the screen's `.withProps()` constant.
+  styles: (_theme, props) => {
+    if (props.variant === "manifest") {
+      return { table: { fontSize: "var(--mantine-font-size-xs)" } };
+    }
+    return {};
+  },
+});