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>

internal/version/version.go

@@ -5,7 +5,7 @@ package version
 import "fmt"
 
 var (
-	Version = "0.9.1"
+	Version = "0.9.2"
 	Commit  = "unknown"
 	Date    = "unknown"
 )