feat: add session creation and ID management with localStorage

- Session store with UUID v4 generation and localStorage persistence
- Session ID in URL params (?session=<id>) for deep linking
- "New Chat" button for creating fresh sessions
- Message history persisted per session
- Session title auto-generated from first user message

Closes #11

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

implementation-plans/issue-011.md

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+---
+---
+
+# Issue #11: Session creation and ID management
+
+**Status:** COMPLETED
+**Issue:** https://git.shahondin1624.de/llm-multiverse/llm-multiverse-ui/issues/11
+**Branch:** `feature/issue-11-session-management`
+
+## Acceptance Criteria
+
+- [x] UUID v4 session ID generated on new chat creation
+- [x] Active session ID persisted in URL params (?session=<id>)
+- [x] Session ID also stored in localStorage for recovery
+- [x] "New Chat" button creates a fresh session
+- [x] Switching sessions updates URL and loads correct message history
+- [x] Session ID passed to ProcessRequest gRPC calls