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)

internal/web/view_test.go

@@ -0,0 +1,104 @@
+package web
+
+import "testing"
+
+func TestPreviewText(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		in   string
+		max  int
+		want string
+	}{
+		{
+			name: "empty input",
+			in:   "",
+			max:  160,
+			want: "",
+		},
+		{
+			name: "whitespace-only input",
+			in:   "   \n\t  ",
+			max:  160,
+			want: "",
+		},
+		{
+			name: "short string unchanged",
+			in:   "A tidy little summary",
+			max:  160,
+			want: "A tidy little summary",
+		},
+		{
+			name: "short single sentence unchanged",
+			in:   "Hello there.",
+			max:  160,
+			want: "Hello there.",
+		},
+		{
+			name: "first sentence taken when more follows",
+			in:   "First sentence. Second sentence that we drop.",
+			max:  160,
+			want: "First sentence.",
+		},
+		{
+			name: "first sentence with question mark",
+			in:   "What is this? It is a preview.",
+			max:  160,
+			want: "What is this?",
+		},
+		{
+			name: "long string truncated on word boundary with ellipsis",
+			// 5 ten-char words past the limit; max cuts mid "ones".
+			in:  "alpha bravo charlie delta echo foxtrot golf hotel india juliet",
+			max: 30,
+			// runes[:30] = "alpha bravo charlie delta echo"; ends exactly on a
+			// word so the next char would be a space — backs off to last space.
+			want: "alpha bravo charlie delta…",
+		},
+		{
+			name: "no mid-word cut",
+			in:   "internationalization frameworks everywhere today",
+			max:  25,
+			// runes[:25] = "internationalization fram" — back off to the space
+			// after the first word; never emit a partial word.
+			want: "internationalization…",
+		},
+		{
+			name: "multibyte safe truncation",
+			// Accented + emoji runes; cutting on rune indices must not split a
+			// multibyte sequence.
+			in:   "café déjà vû señor naïve résumé piñata fiancé",
+			max:  20,
+			want: "café déjà vû señor…",
+		},
+		{
+			name: "multibyte short unchanged",
+			in:   "café señor",
+			max:  160,
+			want: "café señor",
+		},
+		{
+			name: "collapses internal whitespace",
+			in:   "line one\n\n   line two\tline three",
+			max:  160,
+			want: "line one line two line three",
+		},
+		{
+			name: "sentence beyond max falls back to char truncation",
+			in:   "alpha bravo charlie delta echo foxtrot golf. short.",
+			max:  20,
+			want: "alpha bravo charlie…",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := previewText(tt.in, tt.max)
+			if got != tt.want {
+				t.Errorf("previewText(%q, %d) = %q, want %q", tt.in, tt.max, got, tt.want)
+			}
+		})
+	}
+}