Add conflict resolution diff UI and app branding

.project/adrs/adr-005-storage-strategy.md

@@ -0,0 +1,68 @@
+# ADR-005: Storage Strategy
+
+## Status
+Accepted
+
+## Context
+The system needs to store: structured data (users, documents, comments), document content, file attachments, and search indexes. Requirements include: zero external dependencies, backup simplicity, and resistance to data corruption.
+
+## Decision
+**libsql (SQLite) for structured data + content-addressed filesystem for files**
+
+### Implementation
+
+1. **Database (libsql/SQLite)**:
+   - Single file: `data/db.sqlite`
+   - WAL mode for concurrent reads during writes
+   - Embedded — no separate process needed
+   - Backup: `cp db.sqlite db.sqlite.backup` (atomic with WAL checkpoint)
+
+2. **File Storage (Content-Addressed)**:
+   - Directory: `data/files/`
+   - Layout: `data/files/ab/cd/abcdef1234567890abcdef1234567890abcdef12`
+   - SHA-256 hash as filename (hex encoded)
+   - First two characters as subdirectory (for filesystem performance)
+   - Immutable: write-once, never modify
+
+3. **Document Mapping**:
+   - Database stores: `document_path → current_hash`
+   - History table stores all previous hashes
+   - Attachment metadata stores original filename + content hash
+
+## Consequences
+
+### Positive
+- SQLite is the most tested database in the world (billions of deployments)
+- libsql adds replication option without changing the API
+- Content-addressed files are naturally deduplicated
+- Filesystem storage is trivial to backup (rsync, tar)
+- No network dependencies, works offline
+- Single directory contains all state — easy to migrate
+
+### Negative
+- SQLite has write concurrency limits (mitigated by WAL + short transactions)
+- Filesystem storage doesn't scale horizontally without shared storage
+- No built-in file expiration/garbage collection (must implement orphan cleanup)
+- Large attachments can bloat filesystem (mitigated by quotas)
+
+## Alternatives Considered
+
+### PostgreSQL
+- **Pros**: Excellent concurrency, rich feature set, horizontal scaling
+- **Cons**: Requires separate process, more complex backup, network dependency
+- **Rejected**: Overkill for anticipated scale; SQLite is simpler and faster for read-heavy workloads
+
+### Object Storage (S3/MinIO)
+- **Pros**: Unlimited scale, CDN-friendly
+- **Cons**: Network dependency, vendor lock-in, complex local development
+- **Rejected**: Violates zero-dependency and simple backup requirements
+
+### SQLite for Everything (BLOBs)
+- **Pros**: Single file for everything
+- **Cons**: Large BLOBs slow down database, harder to serve directly, backup size bloat
+- **Rejected**: Separation of concerns — database for metadata, filesystem for content
+
+## References
+- SQLite WAL mode: https://sqlite.org/wal.html
+- Content-addressable storage: https://git-scm.com/book/en/v2/Git-Internals-Git-Objects
+- libsql: https://github.com/tursodatabase/libsql