sidebar reorganized: Scope, Plan, Activity — collapsible + resizable

- TaskContextPanel restructured into 3 collapsible sections:
  Scope (graph), Plan (goal + steps + progress), Activity (timeline)
- Collapsed headers show compact live status: 'Graph', 'Step X/N', 'N actions'
- Sections are vertically resizable via drag handles
- Plan section shows goal inline + step list + progress bar
- GoalHeader and PlanProgress no longer rendered separately
- ActivityTimeline header moved to TaskContextPanel

web/src/lib/components/ActivityTimeline.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { activityLog, type ActivityEntry, streaming } from '$lib/stores/chat'
+  import { activityLog, type ActivityEntry } from '$lib/stores/chat'
   import LoaderCircleIcon from '@lucide/svelte/icons/loader-circle'
   import CircleCheckIcon from '@lucide/svelte/icons/circle-check'
   import CircleXIcon from '@lucide/svelte/icons/circle-x'
@@ -41,15 +41,6 @@
 </script>
 
 <div class="flex h-full flex-col">
-  <div class="flex items-center justify-between border-b px-3 py-2">
-    <span class="text-[11px] font-semibold uppercase tracking-wider text-muted-foreground">Activity</span>
-    {#if $streaming}
-      <span class="flex items-center gap-1 text-[11px] text-primary">
-        <LoaderCircleIcon class="size-3 animate-spin" />
-        live
-      </span>
-    {/if}
-  </div>
   <div class="flex-1 overflow-y-auto">
     {#if $activityLog.length === 0}
       <div class="px-3 py-6 text-center text-xs text-muted-foreground">