PRE-RELEASE 0.9.2: frontend rename source→collector, tab layout fit

Rename the collector-picker concept to 'collector' throughout the
frontend so it no longer collides with the ipng_source_tag group-by
(which is labelled 'source'). Affects PageData.Collector, the raw
JSON output key, template labels, and tests. Proto Source field is
untouched (wire-level name used by CLI and aggregator too).

Shrink tab padding/gap/font-size and add window:/filter: labels so
the four tab rows (window, filter, collector, tor) line up and 7+
collectors fit on one line at the 1100px body width.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/frontend/frontend_test.go

@@ -368,10 +368,10 @@ func TestHandlerRaw(t *testing.T) {
 	}
 
 	var result struct {
-		Source  string `json:"source"`
-		Window  string `json:"window"`
-		GroupBy string `json:"group_by"`
-		Entries []struct {
+		Collector string `json:"collector"`
+		Window    string `json:"window"`
+		GroupBy   string `json:"group_by"`
+		Entries   []struct {
 			Label string `json:"label"`
 			Count int64  `json:"count"`
 		} `json:"entries"`
@@ -379,8 +379,8 @@ func TestHandlerRaw(t *testing.T) {
 	if err := json.Unmarshal([]byte(body), &result); err != nil {
 		t.Fatalf("JSON parse: %v\nbody: %s", err, body)
 	}
-	if result.Source != "agg" {
-		t.Errorf("source = %q", result.Source)
+	if result.Collector != "agg" {
+		t.Errorf("collector = %q", result.Collector)
 	}
 	if result.Window != "15m" {
 		t.Errorf("window = %q", result.Window)