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/vite.config.ts

@@ -3,6 +3,7 @@ import type { ProxyOptions } from 'vite'
 import { svelte } from '@sveltejs/vite-plugin-svelte'
 import tailwindcss from '@tailwindcss/vite'
 import { readFileSync, existsSync } from 'fs'
+import { fileURLToPath, URL } from 'node:url'
 import { defineConfig } from 'vite'
 
 // In Docker, VERSION is copied into the build WORKDIR (/build/web/VERSION).
@@ -45,12 +46,27 @@ export default defineConfig({
     __OIKOS_DEV_TOKEN__: JSON.stringify(process.env.OIKOS_API_TOKEN ?? ''),
   },
   resolve: {
-    alias: { $lib: '/src/lib' }
+    alias: {
+      $lib: '/src/lib',
+      // svelte-splitpanes imports SvelteKit's browser-detection module; this
+      // isn't a SvelteKit app, so point it at a plain shim (see the file).
+      // Needs a real filesystem path (not the /src/... shorthand $lib uses)
+      // so esbuild's dependency pre-bundler can resolve it too.
+      '$app/environment': fileURLToPath(new URL('./src/lib/shims/app-environment.ts', import.meta.url))
+    }
   },
   build: {
     outDir: 'dist',
     emptyOutDir: true
   },
+  optimizeDeps: {
+    // svelte-splitpanes imports SvelteKit's $app/environment (aliased above
+    // to a shim), but esbuild's dependency pre-bundler resolves that
+    // differently and fails before the dev server even starts — skip
+    // pre-bundling it so it goes through Vite's normal (alias-aware)
+    // transform pipeline instead.
+    exclude: ['svelte-splitpanes']
+  },
   server: {
     proxy: {
       '/api': authProxy('http://localhost:8090'),