chore: add changelog for v0.5.12

Add v0.5.12 changelog entry with 6 major improvements:
- Batch session deletion (checkbox multi-select, select all, batch delete)
- Visual context length editor (click to edit with custom limits)
- Fix group chat @mention keyboard selection (custom dropdown)
- Chat completion sound (auto-play toggle)
- Improved chat message display (filter empty messages, better compression)
- Improved update mechanism (dynamic path resolution for Homebrew support)

Full i18n support across all 8 languages (zh, en, de, es, fr, ja, ko, pt).

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

packages/client/src/i18n/locales/en.ts

@@ -790,6 +790,12 @@ export default {
 
   // Changelog
   changelog: {
+    new_0_5_12_1: 'Add batch session deletion: checkbox multi-select, select all, and batch delete for improved session management',
+    new_0_5_12_2: 'Add visual context length editor: click token count to open edit modal with custom limit support',
+    new_0_5_12_3: 'Fix group chat @mention keyboard selection: replace NDropdown with custom dropdown supporting keyboard navigation and scroll following',
+    new_0_5_12_4: 'Add chat completion sound: auto-play toggle for better user experience',
+    new_0_5_12_5: 'Improve chat message display: filter empty assistant messages and better history/compression logic',
+    new_0_5_12_6: 'Improve update mechanism: use npm prefix -g for dynamic path resolution supporting Homebrew and non-standard Node.js installations',
     new_0_5_10_1: 'Automate Docker image builds on GitHub release with versioned tags (e.g., :v0.5.10)',
     new_0_5_10_2: 'Add session authorization mode configuration: approvals.mode (off/manual)',
     new_0_5_10_3: 'Add automatic OpenAPI documentation generation (94 endpoints across 24 tags)',