v1.0.0 — Hexagonal architecture rewrite

Restructure the monolithic 252-line main.rs into a 10-crate workspace
with clean hexagonal architecture, swappable adapters, and a production-
ready deployment pipeline.

Backend architecture:
- domain: Canvas, Color/Position/PixelUpdate value objects, port traits
  (CanvasStore, CanvasPersistence, EventBroadcaster), BroadcastEvent
- application: use cases (place_pixel, get_state, save/restore snapshot,
  connect/disconnect), AppState with Arc snapshot cache
- config: AppConfig structs + ConfigSource trait
- api-types: shared DTOs, event name constants
- adapters: config-env, canvas-file, http-axum (rust-embed), socketio,
  websocket — all behind port traits, swappable via feature flags
- server: composition root with graceful shutdown (SIGTERM/SIGINT)

Frontend:
- Transport abstraction: Socket.IO and native WebSocket via VITE_TRANSPORT
- Canvas zoom/pan with mouse wheel, pinch-to-zoom, and +/- buttons
- ImageData rendering (~50x faster than fillRect loop)
- Touch support, responsive CSS scaling, mobile-friendly layout
- OG/Twitter Card meta tags for rich link previews

Production:
- Docker: musl static build on scratch — 2.73MB image
- CI workflows for Gitea and GitHub Actions (fmt, clippy, test, Docker push)
- deploy.sh for private registry
- 39 unit tests across domain and application
- Zero unwraps, zero unsafe, graceful error handling with tracing
- Periodic canvas snapshots with rotation, restored on startup
- All config via environment variables with typed defaults

crates/adapters/canvas-file/src/lib.rs

@@ -0,0 +1,158 @@
+use std::fs;
+use std::io::{BufWriter, Write};
+use std::path::{Path, PathBuf};
+
+use config::SnapshotConfig;
+use domain::ports::CanvasPersistence;
+use domain::{Color, DomainError};
+use tracing::{error, info};
+
+pub struct FileCanvasPersistence {
+    directory: PathBuf,
+    max_snapshots: usize,
+}
+
+impl FileCanvasPersistence {
+    pub fn new(config: &SnapshotConfig) -> Result<Self, DomainError> {
+        let directory = PathBuf::from(&config.directory);
+        fs::create_dir_all(&directory).map_err(|err| {
+            DomainError::Persistence(format!(
+                "Failed to create snapshot directory '{}': {err}",
+                directory.display()
+            ))
+        })?;
+
+        Ok(Self {
+            directory,
+            max_snapshots: config.max_snapshots,
+        })
+    }
+
+    fn snapshot_path(&self, index: usize) -> PathBuf {
+        self.directory.join(format!("canvas_{index}.bin"))
+    }
+
+    fn latest_index_path(&self) -> PathBuf {
+        self.directory.join("latest")
+    }
+
+    fn read_latest_index(&self) -> Option<usize> {
+        let path = self.latest_index_path();
+        fs::read_to_string(&path)
+            .ok()
+            .and_then(|content| content.trim().parse().ok())
+    }
+
+    fn write_latest_index(&self, index: usize) -> Result<(), DomainError> {
+        let path = self.latest_index_path();
+        fs::write(&path, index.to_string())
+            .map_err(|err| DomainError::Persistence(format!("Failed to write latest index: {err}")))
+    }
+
+    fn rotate_and_cleanup(&self, new_index: usize) {
+        if self.max_snapshots == 0 {
+            return;
+        }
+        let oldest_to_keep = new_index.saturating_sub(self.max_snapshots - 1);
+        for stale_index in 0..oldest_to_keep {
+            let stale_path = self.snapshot_path(stale_index);
+            if stale_path.exists()
+                && let Err(err) = fs::remove_file(&stale_path)
+            {
+                error!(
+                    "Failed to remove old snapshot '{}': {err}",
+                    stale_path.display()
+                );
+            }
+        }
+    }
+}
+
+impl CanvasPersistence for FileCanvasPersistence {
+    fn save(&self, pixels: &[Color]) -> Result<(), DomainError> {
+        let next_index = self.read_latest_index().map(|i| i + 1).unwrap_or(0);
+        let path = self.snapshot_path(next_index);
+
+        write_pixels_to_file(&path, pixels)?;
+        self.write_latest_index(next_index)?;
+        self.rotate_and_cleanup(next_index);
+
+        info!(
+            "Saved canvas snapshot #{next_index} to '{}'",
+            path.display()
+        );
+        Ok(())
+    }
+
+    fn load_latest(&self) -> Result<Option<Vec<Color>>, DomainError> {
+        let Some(index) = self.read_latest_index() else {
+            info!("No canvas snapshots found");
+            return Ok(None);
+        };
+
+        let path = self.snapshot_path(index);
+        if !path.exists() {
+            info!("Snapshot file '{}' not found", path.display());
+            return Ok(None);
+        }
+
+        let pixels = read_pixels_from_file(&path)?;
+        info!(
+            "Loaded canvas snapshot #{index} from '{}' ({} pixels)",
+            path.display(),
+            pixels.len()
+        );
+        Ok(Some(pixels))
+    }
+}
+
+fn write_pixels_to_file(path: &Path, pixels: &[Color]) -> Result<(), DomainError> {
+    let file = fs::File::create(path).map_err(|err| {
+        DomainError::Persistence(format!(
+            "Failed to create snapshot '{}': {err}",
+            path.display()
+        ))
+    })?;
+    let mut writer = BufWriter::new(file);
+    for color in pixels {
+        writer
+            .write_all(&color.as_u32().to_ne_bytes())
+            .map_err(|err| {
+                DomainError::Persistence(format!(
+                    "Failed to write snapshot '{}': {err}",
+                    path.display()
+                ))
+            })?;
+    }
+    writer.flush().map_err(|err| {
+        DomainError::Persistence(format!(
+            "Failed to flush snapshot '{}': {err}",
+            path.display()
+        ))
+    })?;
+    Ok(())
+}
+
+fn read_pixels_from_file(path: &Path) -> Result<Vec<Color>, DomainError> {
+    let bytes = fs::read(path).map_err(|err| {
+        DomainError::Persistence(format!(
+            "Failed to read snapshot '{}': {err}",
+            path.display()
+        ))
+    })?;
+
+    if bytes.len() % 4 != 0 {
+        return Err(DomainError::Persistence(format!(
+            "Snapshot '{}' has invalid size: {} bytes (not a multiple of 4)",
+            path.display(),
+            bytes.len()
+        )));
+    }
+
+    let pixels = bytes
+        .chunks_exact(4)
+        .map(|chunk| Color::new(u32::from_ne_bytes([chunk[0], chunk[1], chunk[2], chunk[3]])))
+        .collect();
+
+    Ok(pixels)
+}