feat: Crumbs by Bretzel — self-hostable notes app MVP

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

docs/ARCHITECTURE.md

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+# Architecture
+
+## Overview
+
+Crumbs follows a **local-first** architecture where the client (browser) is the primary data store and the server acts as a sync target and backup.
+
+```
+┌─────────────────────────────┐
+│         Browser (PWA)        │
+│  ┌───────────┐ ┌──────────┐ │
+│  │  Svelte   │ │ IndexedDB│ │
+│  │  Stores   │◄│  (idb)   │ │
+│  └─────┬─────┘ └────┬─────┘ │
+│        │             │       │
+│  ┌─────▼─────────────▼─────┐ │
+│  │      Sync Engine        │ │
+│  │  (LWW CRDT + Queue)    │ │
+│  └──────────┬──────────────┘ │
+└─────────────┼────────────────┘
+              │ HTTP (JSON)
+┌─────────────▼────────────────┐
+│         Server (Node.js)      │
+│  ┌──────────┐ ┌────────────┐ │
+│  │ SvelteKit│ │   SQLite   │ │
+│  │   API    │◄│ (Drizzle)  │ │
+│  └──────────┘ └────────────┘ │
+└──────────────────────────────┘
+```
+
+## Data Flow
+
+### Read Path
+1. User opens app
+2. Svelte stores read from IndexedDB (instant)
+3. Background sync pulls latest from server
+4. If newer data found, IDB is updated, stores react
+
+### Write Path
+1. User creates/edits note
+2. Change written to IndexedDB immediately
+3. Change added to sync queue
+4. Background sync pushes queue to server
+5. Server applies changes with LWW resolution
+
+### Conflict Resolution (LWW)
+For a single-user app with multiple devices:
+- Each note has an `updatedAt` timestamp and `version` number
+- When merging: newer timestamp wins
+- If timestamps match: higher version wins
+- If both match: local version is preferred
+
+## Tech Stack Rationale
+
+### SvelteKit
+- Smallest bundle size of major frameworks
+- Built-in SSR, routing, API routes
+- Svelte 5 runes for reactive state
+- Native service worker support via @vite-pwa/sveltekit
+
+### SQLite + Drizzle
+- Zero-config database (single file)
+- WAL mode for concurrent reads
+- Drizzle provides type-safe queries without ORM overhead
+- Perfect for single-user self-hosted apps
+
+### IndexedDB (idb)
+- Browser-native storage (no size limits like localStorage)
+- Async API doesn't block main thread
+- `idb` library provides Promise-based wrapper
+- Survives browser restarts
+
+### Tailwind CSS 4
+- Utility-first, tree-shakeable
+- Built-in dark mode support (`dark:` variants)
+- New v4 uses Vite plugin (faster builds)
+
+### Argon2
+- Current best practice for password hashing
+- Memory-hard (resists GPU attacks)
+- Winner of the Password Hashing Competition
+
+## Database Schema
+
+See `src/lib/server/db/schema.ts` for the complete Drizzle schema.
+
+Key tables:
+- `users` - Single user (password hash)
+- `sessions` - Auth sessions (30-day TTL)
+- `notes` - Core note data with version tracking
+- `tags` - Unique tag names
+- `note_tags` - Many-to-many note ↔ tag
+- `attachments` - Image file metadata
+- `sync_log` - Sync operation history
+
+## File Organization
+
+```
+src/lib/server/    # Server-only code (DB, auth, attachments)
+src/lib/sync/      # Sync engine (shared types, server + client implementations)
+src/lib/stores/    # Svelte reactive stores (notes, theme, auth, search)
+src/lib/components/ # Svelte UI components
+src/lib/utils/     # Pure utility functions (markdown, tags, colors)
+src/lib/types/     # TypeScript type definitions
+src/routes/api/    # REST API endpoints
+src/routes/        # Page routes (SvelteKit file-based routing)
+```