Add Persistent Chat History, Conversation Workspaces and Professional Chat UI

[abhinavwh17]

Summary

This PR introduces persistent, conversation-based workspaces to NexaMind and updates the application to provide a more polished ChatGPT-style analysis experience.

Each conversation now maintains its own uploaded workbooks, analysis history, questions, answers, and calculation results, allowing users to switch between previous analyses and continue working without losing context after a page refresh or application restart.

Changes

1. Added persistent chat history with a conversation-based workflow.
2. Added a ChatGPT-style sidebar displaying previous conversations.
3. Added New Chat functionality for starting independent analysis workspaces.
4. Each conversation now maintains its own:
5. Uploaded Excel workbooks
6. Dataset/workspace
7. Questions and answers
8. Calculation results and tables
9. Added SQLite-based local persistence for conversations, messages, and workbook metadata.
10. Persisted uploaded workbooks locally so datasets can be reconstructed after an application restart.
11. Added restoration of previous conversations and workbook workspaces when reopening NexaMind.
12. Added support for switching between conversations without mixing workbook data between chats.
13. Added rename and delete conversation functionality.
14. Deleting a conversation also cleans up its associated locally stored workspace.
15. Preserved NexaMind's privacy-first architecture — conversation history and workbook data remain local and are not automatically sent to the AI provider.
16. Added graceful handling for out-of-scope questions.
17. AI plans containing an empty calculations array are now treated as valid out-of-scope responses rather than server failures.
18. Out-of-scope questions now return a normal response instead of 500 Internal Server Error.
19. These interactions are also retained in chat history.
20. Improved Gemini Settings navigation.
21. Added Back to Chat functionality.
22. Users can exit Settings without being forced to update their API key.
23. Updated settings-specific labels and actions for a clearer configuration experience.
24. Redesigned the main application into a more professional financial AI chat interface.
25. Improved conversation sidebar, active-chat state, workbook presentation, chat messages, spacing, cards, tables, and message composer.
26. Added workbook attachment access directly from the chat composer.
27. Improved result export actions with dedicated Excel and PDF icons.
28. Replaced the basic Settings character with a professional SVG settings icon.
29. Kept UI icons dependency-free using inline SVGs.
30. Preserved existing Excel and PDF export functionality.
31. Preserved existing multi-workbook analysis functionality including:

• UNION
• JOIN
• COMPARE
• GROUP_BY
• GROUP_BY_METRICS
• Result-based calculations
• Cross-workbook analysis

32. Existing Gemini API key storage continues to use the local secure credential store.