feat(web): previewText truncation helper for summary cards

One-line lede for list cards: collapses whitespace, prefers the first
sentence within budget, else word-boundary truncation with an ellipsis.
Pure and rune-based (multibyte-safe). Table-driven tests cover empty,
short, first-sentence, word-boundary, and multibyte cases.

internal/web/view.go

@@ -100,6 +100,51 @@ func detailMeta(r store.SummaryRow) string {
 	return strings.Join(parts, " · ")
 }
 
+// previewText renders a one-line lede for a summary card: it collapses internal
+// whitespace, then returns the first sentence when one ends within max runes,
+// otherwise truncates at max runes on a word boundary (never mid-word) and
+// appends an ellipsis. Empty/short input is returned unchanged (no ellipsis).
+// Pure and multibyte-safe — all length work is on runes, not bytes.
+func previewText(s string, max int) string {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return ""
+	}
+	s = strings.Join(strings.Fields(s), " ")
+	runes := []rune(s)
+
+	// Prefer the first sentence when it terminates within the budget.
+	if end := firstSentenceEnd(runes); end > 0 && end <= max {
+		return string(runes[:end])
+	}
+	if len(runes) <= max {
+		return s
+	}
+
+	// Truncate at max runes, then back off to the last word boundary so no
+	// partial word is emitted. Space is single-byte, so the byte-index slice
+	// lands cleanly on a rune boundary.
+	cut := string(runes[:max])
+	if i := strings.LastIndexByte(cut, ' '); i > 0 {
+		cut = cut[:i]
+	}
+	return strings.TrimRight(cut, " ") + "…"
+}
+
+// firstSentenceEnd returns the rune index just past the first sentence
+// terminator (. ! ?) that is followed by whitespace or the end of input, or 0
+// when there is none.
+func firstSentenceEnd(runes []rune) int {
+	for i, r := range runes {
+		if r == '.' || r == '!' || r == '?' {
+			if i+1 == len(runes) || runes[i+1] == ' ' {
+				return i + 1
+			}
+		}
+	}
+	return 0
+}
+
 // videoURL builds the internal detail-page path for a video id.
 func videoURL(videoID string) templ.SafeURL {
 	return templ.SafeURL("/v/" + videoID)