feat: Generate UUID based on file properties (path+filename+mac+mtime)

UUID generation method changed:
- Old: UUID v4 random (uuid::Uuid::new_v4())
- New: SHA256 hash of file properties

Properties used:
- file_path: absolute path to file
- filename: uploaded filename
- MAC address: from ifconfig en0 (ether)
- mtime: file modification time (milliseconds)

Algorithm:
UUID = SHA256(path|filename|mac|mtime)[0..32]

Example:
Input: /path/file.svg|file.svg|aa:bb:cc:dd:ee:ff|1234567890
Output: 32-char hex string

Matches momentry system:
- Uses deterministic UUID (not random)
- Based on file metadata
- Same file = same UUID

Files:
- src/server.rs (line 800-820: UUID generation logic)

src/server.rs

@@ -799,8 +799,35 @@ async fn upload_file(
 
     let file_path = format!("{}/{}", user_dir, filename);
 
-    // Generate file_uuid locally (no external API dependency)
-    let file_uuid = uuid::Uuid::new_v4().to_string().replace('-', "");
+    // Generate file_uuid based on file properties (path + filename + mac + mtime)
+    // Get MAC address
+    let mac_output = std::process::Command::new("ifconfig")
+        .arg("en0")
+        .output()
+        .map(|o| String::from_utf8_lossy(&o.stdout).to_string())
+        .unwrap_or_default();
+    
+    let mac = mac_output
+        .lines()
+        .find(|l| l.contains("ether"))
+        .and_then(|l| l.split_whitespace().nth(1))
+        .unwrap_or("00:00:00:00:00:00");
+
+    // Get file mtime (milliseconds)
+    let mtime = std::fs::metadata(&file_path)
+        .ok()
+        .and_then(|m| m.modified().ok())
+        .and_then(|t| t.duration_since(std::time::UNIX_EPOCH).ok())
+        .map(|d| d.as_millis() as u64)
+        .unwrap_or(0);
+
+    // Generate UUID: SHA256(path|filename|mac|mtime)
+    let input = format!("{}|{}|{}|{}", file_path, filename, mac, mtime);
+    let mut hasher = sha2::Sha256::new();
+    hasher.update(input.as_bytes());
+    let hash = hasher.finalize();
+    let hex = format!("{:x}", hash);
+    let file_uuid = hex[0..32].to_string();
 
     // Save to database (user-specific SQLite)
     let db_path = crate::filetree::FileTree::user_db_path(&user_id);