docs: add ARCHITECTURE.md for offline-first AI system design

[upendra512]

Closes #17

Adds ARCHITECTURE.md documenting the offline-first AI system design for SmartNotes.

Contents:

• Core design principles (offline by default, privacy-first, single-process storage)
• System layers diagram: Electron main/renderer, Web Worker, Ollama daemon
• TypeScript interfaces for all five modules: VaultStorage, EmbeddingWorker, HybridRetriever, RAGEngine, LinkStore
• SQLite schema for notes, FTS5 index, and sqlite-vec embeddings
• Three end-to-end data flow diagrams: note save, semantic search, Q&A
• Performance targets per operation
• Graceful degradation table showing what works without Ollama
• Planned file structure

Checklist:

• Self-review completed
• Documentation updated

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📥 Commits

Reviewing files that changed from the base of the PR and between 4bb58bd and ed68dfa.

📒 Files selected for processing (1)

• ARCHITECTURE.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

This pull request has been automatically marked as stale due to inactivity. It will be closed if no further activity occurs within 7 days.