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/templates/index.html

@@ -2,19 +2,21 @@
 <h1>nginx-logtail</h1>
 
 <div class="tabs">
+  <span class="tabs-label">window:</span>
 {{- range .Windows}}
   <a href="{{.URL}}"{{if .Active}} class="active"{{end}}>{{.Label}}</a>
 {{- end}}
 </div>
 
 <div class="tabs">
+  <span class="tabs-label">filter:</span>
 {{- range .GroupBys}}
   <a href="{{.URL}}"{{if .Active}} class="active"{{end}}>{{.Label}}</a>
 {{- end}}
 </div>
 
 {{if .Targets}}<div class="tabs tabs-targets">
-  <span class="tabs-label">source:</span>
+  <span class="tabs-label">collector:</span>
 {{- range .Targets}}
   <a href="{{.URL}}"{{if .Active}} class="active"{{end}}>{{.Label}}</a>
 {{- end}}
@@ -53,7 +55,7 @@
 
 {{if .Sparkline}}
 <div class="sparkline">
-  <small>{{.Params.WindowS}} trend · by {{.Params.GroupByS}}{{if .Source}} · source: {{.Source}}{{end}}</small>
+  <small>{{.Params.WindowS}} trend · by {{.Params.GroupByS}}{{if .Collector}} · collector: {{.Collector}}{{end}}</small>
   {{.Sparkline}}
 </div>
 {{end}}
@@ -88,7 +90,7 @@
 {{end}}
 
 <footer>
-  {{- if .Source}}source: {{.Source}} · {{end -}}
+  {{- if .Collector}}collector: {{.Collector}} · {{end -}}
   {{fmtCount .TotalCount}} requests · {{.Params.WindowS}} window · by {{.Params.GroupByS}}
   {{- if gt .RefreshSecs 0}} · auto-refresh {{.RefreshSecs}}s{{end}}
 </footer>