feat(chat): implement web chat interface with accessibility

Core implementation:
- chat_app.py: Chainlit entry point with @cl.on_chat_start,
  @cl.on_message (streaming via llm.stream_complete), @cl.on_chat_end
  Reuses Config, KnowledgeBase, ConversationStore, AdminNotifier from src/
  Handles registration completion, post-completion updates, new-child flow

- src/llm.py: add stream_complete() generator (litellm stream=True)
  alongside existing complete(); tests added in tests/test_llm.py

- src/agent/response_parser.py: extract parse_llm_response(),
  apply_updates(), fallback_message() from EmailAgent into shared module
  EmailAgent now delegates to these functions (no logic change)

Chainlit configuration:
- chainlit.toml: telemetry off, German default, custom CSS + JS paths
- chainlit.md: German welcome page with playgroup info

Accessibility (WCAG 2.1 AA):
- public/custom.css: contrast overrides (≥4.5:1), prefers-reduced-motion
  (static "…" replaces animated dots), skip link styles, 100dvh fix
- public/accessibility.js: MutationObserver injects aria-live="polite"
  on message list, focus management after agent replies, skip link element

Other:
- .gitignore: add .chainlit/ (Chainlit runtime, auto-generated)
- openspec/config.yaml: populate context field with tech stack
- openspec/changes/implement-web-chat/tasks.md: mark completed tasks

95 tests pass.

https://claude.ai/code/session_01SUWzMzFvSfWiHXA2p6rPg9

chainlit.toml

@@ -0,0 +1,18 @@
+[project]
+# Project name shown in browser title / header
+name = "Spielgruppe Pumuckl"
+# Never send usage data to Chainlit cloud
+enable_telemetry = false
+
+[UI]
+name = "Spielgruppe Pumuckl"
+# German is the default language; the agent switches automatically to English
+# if the parent writes in English
+default_language = "de"
+# Accessibility overrides on top of the default Chainlit theme
+custom_css = "/public/custom.css"
+# Accessibility JS: ARIA live region, focus management, skip-to-content link
+custom_js = "/public/accessibility.js"
+
+[meta]
+generated_by = "2.x"