feat(web): resizable panels via svelte-splitpanes + Claude-style composer

Replace hand-rolled pointer-resize logic (TaskContextPanel's 3-way vertical
split, SessionChatWindow's rail, ChatThread's message/input split) with
svelte-splitpanes, themed onto the app's existing border/primary tokens.

- TaskContextPanel: Scope/Plan/Event-log sections collapse to a fixed header
  height and restore their last size on reopen.
- ChatThread: input area is now a separate resizable pane, clamped to a
  measured one-line minimum and a 45% max, instead of a fixed max-h textarea.
- Send button restyled to sit inside the input's corner (Claude-style),
  swapping the up-arrow for a corner-down-left return icon.
- Adds a $app/environment shim + optimizeDeps exclude, since
  svelte-splitpanes assumes SvelteKit and this is a plain Vite app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/components/SessionChatWindow.svelte

@@ -6,6 +6,7 @@
   // these can be open (and independently live) at once without the "which
   // one's on screen" guarding the main page's singleton stores need.
   import { onDestroy, onMount } from 'svelte'
+  import { Pane, Splitpanes } from 'svelte-splitpanes'
   import { chatFor, loadSessionChat, sendSessionMessage, cancelSessionStream, stopSessionPolling, dismissError, chatErrors } from '$lib/stores/chat'
   import ChatThread from '$lib/components/ChatThread.svelte'
   import TaskContextPanel from '$lib/components/TaskContextPanel.svelte'
@@ -30,29 +31,9 @@
 
   onDestroy(() => stopSessionPolling(sessionId))
 
-  // Resizable right rail — same behavior as Chat.svelte's, sized smaller by
-  // default since task windows open narrower than the full page.
-  const RAIL_MIN = 220
-  const RAIL_MAX = 480
-  let railWidth = $state(260)
-  let resizing = $state(false)
-
-  function startResize(e: PointerEvent) {
-    e.preventDefault()
-    resizing = true
-    const startX = e.clientX
-    const startW = railWidth
-    function move(ev: PointerEvent) {
-      railWidth = Math.min(RAIL_MAX, Math.max(RAIL_MIN, startW + (startX - ev.clientX)))
-    }
-    function up() {
-      resizing = false
-      window.removeEventListener('pointermove', move)
-      window.removeEventListener('pointerup', up)
-    }
-    window.addEventListener('pointermove', move)
-    window.addEventListener('pointerup', up)
-  }
+  // Resizable right rail — sized smaller by default since task windows open
+  // narrower than the full page.
+  let railSize = $state(24)
 </script>
 
 <div class="flex h-full min-h-0">
@@ -64,34 +45,23 @@
       <p class="text-xs text-muted-foreground/70">It may have been deleted.</p>
     </div>
   {:else}
-    <ChatThread
-      messages={$chatMessages}
-      streaming={$chatStreaming}
-      connectionState={$chatConnectionState}
-      error={$chatError}
-      chatErrors={$chatErrors}
-      onSend={(text) => sendSessionMessage(sessionId, text)}
-      onCancel={() => cancelSessionStream(sessionId)}
-      onReconnect={() => loadSessionChat(sessionId)}
-      onDismissError={dismissError}
-    />
-
-    <div class="flex shrink-0" style="width: {railWidth}px">
-      <button
-        type="button"
-        class="group/rz relative w-1.5 shrink-0 cursor-col-resize touch-none"
-        onpointerdown={startResize}
-        aria-label="Resize task panel"
-      >
-        <span
-          class="absolute inset-y-0 left-1/2 w-px -translate-x-1/2 transition-colors {resizing
-            ? 'bg-primary/60'
-            : 'bg-border group-hover/rz:bg-primary/50'}"
-        ></span>
-      </button>
-      <div class="flex min-w-0 flex-1 flex-col">
+    <Splitpanes theme="oikos-theme" dblClickSplitter={false}>
+      <Pane>
+        <ChatThread
+          messages={$chatMessages}
+          streaming={$chatStreaming}
+          connectionState={$chatConnectionState}
+          error={$chatError}
+          chatErrors={$chatErrors}
+          onSend={(text) => sendSessionMessage(sessionId, text)}
+          onCancel={() => cancelSessionStream(sessionId)}
+          onReconnect={() => loadSessionChat(sessionId)}
+          onDismissError={dismissError}
+        />
+      </Pane>
+      <Pane bind:size={railSize} minSize={18} maxSize={40}>
         <TaskContextPanel {sessionId} />
-      </div>
-    </div>
+      </Pane>
+    </Splitpanes>
   {/if}
 </div>