feat: add session history sidebar with delete and navigation

- SessionSidebar component listing past sessions sorted by recency
- Session title preview and relative date display
- Click to switch sessions, delete with confirmation
- Added deleteSession method to session store
- Integrated sidebar into chat page layout

Closes #12

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

implementation-plans/issue-012.md

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+---
+---
+
+# Issue #12: Session history sidebar
+
+**Status:** COMPLETED
+**Issue:** https://git.shahondin1624.de/llm-multiverse/llm-multiverse-ui/issues/12
+**Branch:** `feature/issue-12-session-sidebar`
+
+## Acceptance Criteria
+
+- [x] Left sidebar component listing past sessions
+- [x] Each entry shows timestamp and first-message preview
+- [x] Click on a session loads its message history
+- [x] Session message history persisted in localStorage
+- [x] Sessions sorted by most recent first
+- [x] Delete session option (with confirmation)