feat(web): segment watched/skipped action toggles

Watched and skipped are mutually exclusive (the store clears one when the other
is set), but rendered as three independent-looking buttons the exclusivity was
invisible. Group watched|skipped into a single segmented control and keep Saved
apart as an independent toggle (UX review C5). HTMX posting and the active/✓/
aria-pressed semantics are unchanged; extracted a shared actionButton component.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/handlers_test.go

@@ -234,6 +234,9 @@ func TestDetailRendersHighlightsAndTakeaways(t *testing.T) {
 	require.Contains(t, html, "takeaway one")
 	require.Contains(t, html, `id="action-buttons"`, "action button group present")
 	require.Contains(t, html, "Watched")
+	require.Contains(t, html, `class="segmented"`, "watched/skipped render as a segmented control (UX review C5)")
+	require.Less(t, strings.Index(html, "Skipped"), strings.Index(html, "Saved"),
+		"Saved sits after the watched/skipped segment")
 	require.Contains(t, html, "← Summaries", "back link to the list (UX review C3)")
 
 	// The detail page leads with the attention-saving payload (UX review A8):