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/package.json

@@ -45,6 +45,7 @@
     "d3-force": "^3.0.0",
     "dompurify": "^3.4.11",
     "marked": "^18.0.5",
+    "svelte-splitpanes": "^8.0.12",
     "tailwind-merge": "^3.6.0",
     "uplot": "^1.6.32"
   }